those neo motors are very good, and very tolerant of "upping" the voltage, I ran one in a 36" crash tender for a while, on 2 x 9.6v nimh packs, it was very good, but needed water cooling. Graupner motors state a voltage, but also a voltage range, for speed tgry and get up to the top end of this, but the run time will decrease, its a play off, short run, but fast! long run, slower!
Totally agree with nick's post, lipos will get it shifting, but require maintenance. I would stick with minhs till your experience is better, as you can run them flat, you cant run a lipo flat. One thing though, that neo motor is a bit amp hungry, make sure you are using decent connectors on the batteries otherwise they will get hot