With servos your motors rated speed determines your max velocity, or rather it should. There are several things which could affect this such as you running a lower voltage than the rated speed is given at. The pulse rate needed at that speed could be too high for the kernel.
What is the rated speed of your servos and is there any gearing between motor and ballscrew and what pitch is the ballscrew?
As for the version upgrade, what problems do you get?
Hood