[neallewis]

8.5 should be fine. The gearbox will handle it for sure.

I run a 13.5 in mine, but I'm only running it on small indoor slippy tracks at the moment. It's as competitive as any modern car in the same track.

On a super high-grip track you may struggle to get it competitive, but on a lowish traction track it will work just fine. There is lots of rear traction and its difficult to stop it under-steering on power. I've added quite a bit of weight to the front of mine, and reduced the front droop with internal shock limiters.
I'll be running mine with a hotter motor (say 10.5) when we move indoors for the winter. I'll also be running it in the York club 540 sealed can class next indoor season. It's a car that puts a smile on my face, just fun to drive.