I would think you'd want a 540-7s (5070 rpm x 7.2v = 36,504 rpm). that would be pretty fast and similar in performance but slightly slower than the mamba max which is 5700. The 8s would give you a max rpm of 31,903. Both examples assume a 7.2v/6-cell pack. A 7-cell or a Li-Po pack would effect the numbers and likely raise the rpm.
Here is the link to the motor specs for the 540 & 550 sized Nemesis/Feigao/Wanderer motors:
http://www.offshoreelectrics.com/fei...tors_chart.htm