What kind of machine is this? Unless it's very large, you should be able to get all the speed you need with the proper steppers, and do it for a lot less money.
What motors, controllers, and power supplies do you have now? Speed problems are most often the result of using motors that are too small, and/or controllers that have limited voltage/current capability, and/or power supplies that do not provide enough voltage/current. Switching to servos will not solve your problem any more than steppers will, if the whole system is poorly designed.
Regards,
Ray L.