On a certain Pink Poster's Blog, Abusepuppy had a remake of the Nid codex as a pet project, for reason's unknown.
One good idea he had (besides some re-costing on units) was to make the synapse do things besides let the nids play. Aura's that hand out a universal special rule, or give immunity to force weapons, or just normal instant death (already covered I see). I do think thinking up of a way for a synapse biomorph to act like Marks of Chaos would be a great way to tweak the codex without having too much in the way rules conflict, or cries of OP.
Personally, I think the only over-costed unit in the codex is Carnifex and Tyranofex.