I'm thinking about some enhanced rules for Synapse effect when GM'ing those wacky Nids. Primarily it's give to give some nicer obvious RP effects, secondarily it's a chance to include some glaring (only to me no doubt) ommisions.
Synapse rating. At the start of combat write down the synapse rating of the Tyranid opposition. This is measure of number and power of Synapse creatures in the area. Select the most powerful creature with the Synapse trait, add it's WP and INT bonuses together. Then add the INT of every other Synapse creature in the vicinity.
EG. If a force of Tyranids includes only 'gaunt' sepcies and a group of three Tyranid warriors the Synapse rating would be 11 (5WP+2INT+(2x2INT)).
Exerting synapse. Synapse points are spent to increase the focus of the connection and teamwork of groups of Tyanid creatures. For each group (and distinct team of like creatures), horde and single creature may have Synapse points spent at the start of combat to give them any of the effects listed below. However, each group, horde or creature must have 'Synaptic Link' spent for them before any other points are spent on them. Effectively the points refresh every turn and all spends are for the duration of that turn but normally the points are spent each turn in the same way as the last turn unless something happens to effect the amount of points. The GM could also decide that significant changes in battle conditions could have the Synapse (effectively the most powerful Synapse creature) make a WP roll to change the points distribution.
Synaptic Link, cost 1. The creatures are fully linked to others Tyranid creatures in the Area. Effects, the Tyranid creatures gain the talent Double Team, If any Tyranid creature with Synaptic Link that is can see a character then all Nids with Synaptic Link are aware of them, although direct attack may still suffer the -30 penalty to hit a target they can't see although some blast weapons might make direct targeting unnescesary. Perception tests made by a Nid with Synaptic link are assisted (i.e. +10) by any other Synaptic linked creatures with potential los to the target.
Creatures within synapse range that do not have Synaptic Link established only overcome their instinctual behaviour.
e.g. In the above example the Three warrior group could establish a Synaptic link with 10 other creatures, groups or Hordes.
Tyranid Tide, cost 1. Groups of creatures move and flow around attacks like water. Hordes with this ability have the ability to dodge attacks.
Synapse resistance, cost 1. Tyranid creatures with this ability gain a bonus to resist mind controlling or fear effects of +10 for every creature with the Synapsetrait linked to them (creatures with unnatural WP donate a bonus of 10 x thei Unnatural WP trait).
That's all I've got time to do at the moment, I'll think of some more later. I know this is going to be too complicated for most GM's, in fact it looks a longer written down than I was expecting but I think the core mechanics are pretty simple.