My Tree retro-fit works great the the VFD. Just recently purchased an interface that will allow the software to control F-R, speed and braking, if I see a need , then will add an encoder to co-ordinate x-y-z with the spindle.
I do not see any performance loss, can run more RPM, my varible drive can be used for any torque issues.
The only draw back I see is the VFD makes some noise that screws with my house extension phone if I'm close to the mill.
Adobe (old as dirt)