Hi Sam, I didn't know Toyo did a 255/40 R888, but even if they did, I believe you get most neutral handling out of the car using a square set up, hence 285/35 all round. It's the same reason but from different causes. On the MPSS, the 255s don't grip well enough hence understeer, on the R888s, the 315s grip too well, hence ultimately, understeer, albeit at much higher grip levels than the MPSS.
So to cut a long story short, get 285 R888s in the absence of MPSS availability!
David