I got my rc8 up to 70mph with the 1.5d on 5s moderately geared. I think you could actually break 80mph though if you get the setup (lowered, shocks, springs, etc) of the car right. I never even got to full throttle. Mine was wheeling all over the place. The bigger issue is keeping the tires on the rims. I could only make about 3 passes before the tires start blowing off the rims.

I used the tires you have in your picture and they held up pretty well after I reinforced the glue. Foams would probably work best.