wrote:
What's the highest step frequency one can expect from popular CNC
software these days? 100kHz? 200kHz?

If you want to look at a complete system with high performance, check
out http://www.cncteknix.com They have controllers which can handle
40,000 steps/sec per channel (max 4 channels), with high current
capability, and G-code interpreter all included.
Regards, Ian Kirby.