17.5 w/ 2s LiPo is faster than 13.5 w/ 4-cell NiMH hands down. No contest.
21.5 w/ 2s LiPo may well be closer...would be interested in seeing.
Why has no one suggested 13.5 w/ 1s LiPo and 17.5 w/ 4-cell NiMH? Give up about 25% in voltage, gain about 25% in kv...I think this would be the closest combo yet.