The G100 can put out 1Mhz per axis so I don't think it will be a problem.
If you motor was a stepper motor [I know its a servo] with 200 step/rev and your drive uses 10ustep [micro step] then you have 2000step/rev. 1million / 2000=500Rev/sec. or 30,000RPM.. I think it would have it covered.
In your case w/ a 1500 line encoder, it puts out 6000 index's per rev [ in Quad] so that limit's it to 166.6 rev/sec input into the Gecko or..10,000rpm.. so thats your limit.. and I still think its got it covered
In reality.. its going to be darn tuff to reach the G100's limit's.