I'd be putting a 10.5 in (maybe even a 13.5) on the sort of surface you describe. A slower motor geared for speed will always be easier to drive on very slippery surfaces than a punchier motor. Remember a car that is pointing in the correct direction all the time will always be faster round a lap than a car that is rapid between spins
I normally run an Speed Passion V3.0 8.5R in my 2WD on all but the highest grip / fastest tracks where I consider a 6.5 to be the absolute maximum.