You should manage to get the max velocity that your voltage/motors allow, not sure what that is as dont think you actually said what the rated voltage/rpm was but we know what voltage you are using so can work it out near enough. One thing though, is the 50v you mentioned earlier AC or DC?
What size machine is this on?
What are ALL the specs of the motors?
It could be the Geckos are just not capable enough for the current requirements of the motor but until you post the specs I cant really say.
One last thing, to me accel is more important than Velocity, Velocity is nice as well so if you can get both all the better but you may be farting against thunder to get both depending on how well the machine/motors and drives are matched.
Hood