I would say the rb 12 is faster than mt12,but if your local track is med. or small the wouldn't make any differance what motor you used.
for big track yes get the fast motor
You can change gearing and is a big part of speeding
A guy at our local track runs one in his OB4 and it is balistic! He does run an 1/8th scale pipe though and the low end power is insane compared to my MT12! Another guy at out track runs a V12 with the lowest available NTC3 gears and it flies too. My MT12 is fast but I think it coulc be faster.