Interesting idea as i was considering using a servo motor for my next machine, i know the use them on big vmc's but its finding a servo motor that will have a good top rpm as a lot of brushless servos only go to around 3-4k mark failing that you could always 1:2 belt gearing to get 6-8k out of it but i suppose your sacrificing torque..

There is another option, using a vector motor... but i don't know that much about them (yet anyways)