Once you start getting skipped steps due to reduced stepper tq at higher speeds you might want to start playing around with lower flute counts. This will lower your feed speeds and let you push the cut until you are router power limited.
Ive been playing around with single flute compression cutters in oak to minimize feed speed and maximize router rpm. The compression also helps with axial forces and floor/top finishes.
I gotta start using the Modkita again to start getting real k factor.