Nice question For circuit work using 8" wide rims I've found 225 medium compound R tyres fine, thats with the S30 and say 250whp and turbo torque. For ordinary road use 7" rims with say 205 tyres would be fine.
Taking into account the practical limitations imposed by the authorities and car manufacturers, for circuit work I'd go for the widest that will fit. The wider the tyre the softer the compound you can use the more grip you will get, its that simple. But the rim width has to be appropriate to the tyre size, some people try to run wide tyres on narrow rims, not sensible.
If/when my 500whp project eventuates, I'm planning on 10" wide rims for circuit work on probably a 16" diameter wheel. I consider 10" a minimum requirement with that sort of power.