Yeah, that sounds too small to me too... Hopefully won't look 'stretched'! Seems like anything from 295-305 would be ideal. Makes sense that 315 would just be adding weight for an NA 370z. Re 285, guessing Mike is going with what's available through his tire sponsor
BTW, here is the best deal I've been able to find on R888s! If anyone has seen better pricing, SPEAK UP !
TOYO Proxes R888 305 35ZR18 101Y @ Tire Deals 4 Less