I like the 11.5 just because it would push it out a little more, trying to get every last MM out of that tread width... In Japan the RE71R comes in a 295/30/18, which would be the best of all worlds, a square setup on 18x11s with those would be my first choice for the Valiant. Realistically though I can't imagine anything being much faster than these 275 RE71R on the 10.5s all around. I'm having trouble trusting the car to the limit since it's so far past what I'm used to. You can see in the pic below the 275 RE71R on front and the 315 Rival on rear...

Current Autocross ranking for 200 TW tires:

Bridgestone RE-71R
BFG Rival S
Dunlop Z2 Star Spec
Hankook RS3 V2
Dunlop Z2 non star spec
"New compound" Falken RT615K
Original BFG Rival
Dunlop Z1
300 Treadwear Michelin Super Sports.

I would move the Hankook up above the Z2 star spec if it's really hot, and down below the Falken if it's very cold. I would move the supersports up the rankings for a track day where I'm running lots of laps in a row. There are the new Toyo R1R and Kumho V720 coming out, rumors put them near the Hankook, definately not Re-71R or Rival S fast...