Simply put, the Tyranids went past the point of "best efficiency". Continuous tweaks to make the list as balanced as possible meant that points were directed away from areas that had previously provided the list with overall strength whilst still providing flexibility.
The list I ran at the ETC was as follows:
HQ 1: Tervigon (160), Cluster Spines (0), Dominion (0), Catalyst (15), Onslaught (15), Toxin Sacs (10), Adrenal Glands (10)  (Warlord)
Elite 1: 2 Zoanthropes 
Elite 2: 2 Zoanthropes 
Elite 3: Doom of Malan’tai (90), Transport 1 
Troop 1: Tervigon (160), Cluster Spines (0), Dominion (0), Catalyst (15), Onslaught (15), Toxin Sacs (10) 
Troop 2: Tervigon (160), Cluster Spines (0), Dominion (0), Catalyst (15), Onslaught (15), Toxin Sacs (10) 
Troop 3: 10 Termagants - 50
Troop 4: 10 Termagants - 50
FA1: 28 Gargoyles (168), Toxin Sacs (28) 
FA2: 28 Gargoyles (168), Toxin Sacs (28) 
FA3: 28 Gargoyles (168), Toxin Sacs (28) 
HS1: 2 Biovores 
HS2: 2 Biovores 
Transport 1: Mycetic Spore (40), Transports Elite 3 
ARMY TOTAL [1848pts]
The army could cover the board (through fast moving gargoyles who would move 12" and run up to 6" per turn, fully spaced at 2" x 84 models plus backfield covered by Gaunts, spawned or otherwise. remember the first ~15" don't require cover due to 36" flier movement and base size) if it chose, dictating where fliers would arrive and controlling their movement. A skilful player was able to mitigate this and reduce the effectiveness of this through carefully placed charges. For example, in game 1 I rushed to position in turn 1 due to the presence of 2 heldrakes and 2 Night scythes. I was promptly charged by a detached Destroyer Lord who dragged a unit of Gargoyles out of position enough to bring in 2 fliers on turn 2. A similar thing happened in Game 6 when I charged 2 units of Wraiths and a Scarab Farm with all my gargoyles and spawned Gaunts (I'd reduced them all to T2 and was going to instant kill with S4 guns then charges). This of course dramatically reduced my board control, especially when I massively failed to kill any of the opponents models! (Slight exaggeration, I killed about 3 bases of Scarabs :( )
The result of this was that, in both games, I no longer had control of the board and the angle of attack for fliers. They were free to engage and I lost both of these example games. The concept was solid against 4/6 players and against playtest opponents and tournament players alike.
But for this list, this was the only way to deal with fliers. Control the board and dictate their movement, access to the game and what they could kill. I chose not to use Flyrants with dual Devourers as I dislike their resilience. They are too easy to kill once grounded and unlike say Fateweaver FMC Demons, have no way of manipulating that critical failed grounding check. With only 4 wounds as well I found in testing that they simply died. Josh, our captain agreed with this assessment, as did the team members I discussed it with. On the other hand, they would have provided me with some Skyfire shooting and a fast pressure unit to harass the enemy. In the end, they simply cost too much.
Again, I return to the 'best efficiency' point - in tournaments and playtesting I had primarily only used 60 gargoyles not the 84 I ended up using in the ETC. These were supported by The Swarmlord whose extended synaptic control kept them from fleeing/fearless and could also grant Furious Charge so they might bring down vehicles and gain rerolls on Poison against T4 targets.
The Swarmlord also provided a significant midfield threat and counter assault element and with Biomancy, telepathy and telekinetic support from the army and himself, is extremely resilient. Crazily so... Iron Arm, Endurance, TK Dome or Invisibility and Endurance make for a very resilient bastard.
His ability to Instant Death anything from one wound (barring Eternal Warriors) is extremely useful when playing MCs or engaging DLords and the like.
His replacement was a Tervigon. Whilst I only gave up Warlord Secondary in 2/6 games this was more to do with my defensive play with him than the opponents difficulty in killing him. A 3rd Tervigon did mean more Gaunts and did push the board control concept further, he also provided more psychic power and synapse.
Against The Swarmlord however, this pales in usefulness.. Especially when the Swarmie build HAD three Tervigons anyway.
So why the change?
The Tau release and early use of massed Fire Warriors and Kroot, castles with an Ethereal meant that I needed a decent tool to counter the Storm of Shit coming into the Gargoyles when they pushed to pin in the opponent into a defensive position in their own deployment zone. The Gargoyles simply could not charge or stay alive (despite the synergy of FNP and TK Dome) for the length of the game. Remember, these guys were in position from Turn 1 to be shot in the Storm of Shit range.
The counter to this castling of infantry was either the use of triple Mawlocs (!) or a cheaper more utility based unit like Biovores. The Biovores had the S4 and AP to cause a real pain to the Tau infantry castles and meant that I could hold back with gargoyles for turns 1-3 before launching the Gargoyles forwards.
Whilst this is a perfectly valid tactic and from this position the Tau player would struggle to kill all the models (we've built up two to three turns of Gaunt spawning too) in the army and I have the relative speed to get to almost any objective in the remaining two-four turns (2-3 for sure), the pairing process and the composition of the Tau armies present at the ETC were not what I had prepared the list for (with the exception of Gonyo's/USA's Tau which with hindsight I perhaps should have gone for). In addition, the rest of the team had Tau either covered or felt we had better match ups...indeed , I felt that I had better match ups elsewhere.
So in the end, the move to "balance" the army for the Tau presence meant that the Biovores were not all that useful - they killed some Gaunts on objectives in Game 5, they attempted to glance Manticores side armour, they killed a couple of cultists, they did nothing at all versus FMC spam, they utterly failed vs Chronometron 2+ saves...so we have 180 points that pushed the list past its point.
Lets consider the alternatives. We can bring in Furious Charge on all gargoyles and on all Gaunts (via Tervigon Adrenal Glands) and grab another Zoanthrope or increase the size of the starting Gaunt units and finish off all 3 gargoyle units to max.
I could also buy a 4th Tervigon into HQ, maybe a slight points tweak based off the 180 to bring in a 5 & 6th troop unit to start the game.
In including the Biovores, I reduced the offensive capability of the rest of the army, the components of the army that I actually used at the ETC and therefore my game plan effectively changed from board control and killing the enemy to controlling the board until they killed me. A stalling game if you will.
In a war game, an inability to kill the enemy is a real problem. Whilst the list has a demonstrable ability to kill, it struggles against some enemies and in this manner I do wonder if the Swarmlord build would have done better in the games. It's easy to criticise with hindsight, but after any tournament it is a very useful exercise to look back and analyse performance.
Moving forward - Nids will take a back seat to Tau, Demons and maybe even Ravenwing....armies with less than 180 models on just MY side of the board.
I eagerly anticipate new Tyranids which are rumoured to be closer than we think! Exciting stuff...
Will I play Nids next year at the ETC? Who knows, I know at least one other member do the squad wants to run them and if they turn out really well in the new book...more than that! That's just the way of it. It's nice in the current squad as I can think of at least 4 other players in the squad that COULD run Nids - you won't get pigeonholed as "the" Nid player but you also have competition!
Peace- I'll be back later in the week with my plans for the next three months, each month a different tournament!