If you're rotating things at 20,000 rpm, they need to be very well-balanced, or there will be a lot of vibration that would throw off any measurements you're trying to make. It can also be dangerous. If these are turned parts, you need to make sure they are held the same way they were when they were made; refixturing can cause the balance to shift.
I'm not sure what sorts of measurements require that much speed, but normal spindles are not meant for this sort of use. Look into high-speed lathes if this is what you're trying to do - and maybe try to figure out a way to make your measurements at about a tenth that much speed. If this isn't your area of expertise, consult with a mechanical engineer to get a little reality on the situation - it doesn't really sound feasible to me.