I ran top level NiMH2 on a large dirt banked oval with a 4.5 sintered rotor. Only got 40 laps of a 50 lap race. Batteries came of 165 F or so... way too hot.
These were Team Express packs only a month or two old - very low IR, very good packs, and they got too hot. I think a high capacity LIPO would be a better choice. By high capacity I mean 6,000 or more, preferrably 8,000.