If the German PRO10 rules are anything to go by, definitely 10th pan cars. They run 5300mAh 2S lipos and no motor limit, so it's 2.5 and 3 turn brushless motors they are using.
They have to be carefull not to dump the lipo before race time is up and are currently lapping around the same speed as 1/8th scale onroad cars on open tracks.
I think if you geared the hell out of it and ran it in a straight line anything else would have a bloody hard time catching up...