A couple questions:
1) What model were you thinking of to replace the Biovores?
2) What attack bugs would you recommend and what are their chances vs Tau?
3) Has anyone actually won with this codex yet? If so, can you post your list? If you haven't played, what do you think will win?
1) On paper at least, vs Tau the Exocrine should work really well. The large blast will get squads, or take out battlesuits with equal ease. It also can pop light to medium tanks, given it's S7. And it needn't replace all of them
2) In my last game vs Tau, once they made it in my Hormogaunts minced Fire Warriors. Using venomthropes or cover enough should make it in if you plan ahead. Raveners also work well as they are sickeningly fast, but you need to utilise cover really well. Basically, any attack bug is going to have a hard time, unless you can make something else look like more of a threat. Take the faster ones though, as ideally you only want to face 1, 2 turns tops of shooting. Gargoyles are a very good, cheap distraction unit that look like enough of a threat that they should draw fire. Or alternatively take out Riptides
3) In the next couple of weeks I plan to start testing my nids, but I'm confident I shall do at least as well as I used to, and hoping for better as I can now afford more of my bugs
A couple comments:
1) I actually had no problem with synapse because by the time it mattered I was dead.
2) As I mathhammer this codex I don't see any reliable tank popping or assault units. Obviously other people see something I don't so I'd love it if you'd share.
Nids have always been better at popping tanks in person. Now they have a couple more options for at range, and a bunch more haywire (which many bigger bugs can take for 'pop then charge') it should be even easier...