Nicolas,
I am using a light duty from Automation Direct that is rated for 4800 rpm, but my mill tops out at 2500 rpm (at least currently).
My guess is that the speed rating is a function of the electronics ability to read the pulses without missing any.
I would suggest that you ask AutomationDirect or see if they can check with the manufacturer of the encoder to see if the limit is the electronics or the bearings and construction materials.
My encoder seems to be working correctly, but have not done any tapping yet. Still running milling and drilling cycles.
Also trying to figure out the inches per minute, depth of cut and depth per tooth that I can use on the PM25 without breaking tools (2 so far, one from not clearing a hold down screw the other not having the set screw tight enough so that it sucked the end mill out of the TTS holder). Live and buy replacements tools.
Maybe someone that has a high speed spindle can tell you what is on there machine.
Chuck