My 420 with 8" rears effortlessly spins up it's rear tyres on the road. In most situations though, I wouldn't be pushing the car anywhere near the physical limits of grip on the road and so 8" works just fine - by which I mean the balance of the car is harder to determine and less relevant during road use IMO. I think the issue is that they don't get any meaningful temperature on the road and so especially over crests, bumps and general undulations it can be quite interesting... I wouldn't want less rear grip... On track with hot tyres it's completely planted and I have been adjusting the car to take of a bit of rear grip to reduce understeer. What I couldn't say is whether the 6" rears (with equivalent narrower tyres) would warm up more easily during road use. I doubt it TBH David.