Following just to learn. I really cant contribute much to this discussion but I'm interested in what you come up with.
I built a 3 axis router that used a 2.2KW, 24K RPM spindle. The machine was plenty stiff and using it I found that my limiting factor in how fast I could cut was the HP of my spindle. I never approached max VFD current until I used a spoilboard surfacing bit. Then I had to be careful with my feedrate and step size or it would trip out on overcurrent.