Sumtak was taken over by Renco I believe and they were in turn taken over by Heidenhain.
For DC servo's you can usually reckon on 2krpm, max, and BLDC anywhere up to 6krpm.
If you have the max voltage rating for them, you could back feed them with a motor of some kind and measure the generated voltage at recorded the RPM, this will give you the maximum rpm if you extrapolate the numbers.
Al.