Stan,
A SS will provide for an increase in the number of pulses that can be provided to the drives. The question is ....... are the drives and motors capable of using the pulses. The SS is generating the signal and not the PC/PP. The signal quality may be better at higher rates than from the pc and motor preformance will be improved.

The problem I am having is that you are trying to make a correlation of how a stepper motor will preform  running unloaded as to compared to one
loaded ( hooked to something ). That makes no sense at all from a practical point or theory. There is a world of difference. And depending on the application you should care about velocity and accel at this time. Well maybe not, since you already have the motor and you got what you got. You could use a motor sizing program and quickly see what the affects of varing  conditions provided you have known motor characteristics ( which you don't have ) and would see how velocity and acceleration comes into play at a given load.
Why do you think manufactures provide preformance motor curves?

Like Gerry, I get  the same performance in motor tuning and running g-code also.

Unloaded motor  you are mention IPM, yes relative is a sense only, since when a load is attached you will find that if your operating the stepper motor past some point of say max power the feedrate is no longer linear  because the torque is decreasing and as such the feedrate will no longer be linear
and any addition increase to the feedrate will be less than commanded. In fact you can almost generate a curve for your motor until it stalls based on the load!

RICH