The Kemper rotary has a ramp up and down speed. You want it to be controllable, correct? Different Leslie models ramp up and down at different speeds, such as a 147, 122, 125, etc... Those can only be slightly adjusted and mostly via belt tension on the lower rotor only. I think Kemper has stated that the ramp up and down works just like a real Leslie. But I have wondered which real Leslie does is mimic? It can't mimic more than one model as it is currently implemented.
In the end, I am in favor separate, adjustable controls for the upper and lower frequencies. This is what Kemper should implement if they want the rotary effect to match real rotary speakers.
One of the other features on real Leslie's was a brake. This was another key to achieving the organic feel of a real spinning speaker. Speeding up and slowing down.
Playing it like an instrument.