you want to erase the effects of bumps first of all and much more important:
When generating downforce you will of course compress the springs lowering rideheight in this .So your measurements of force are much easier when rideheights are controlled and steady so will be your reproduceability-leaving the tyre squash as the variable in the equation.
to generate a aeromap this seems quite a good idea.additionally you can easily change rideheights front and rear left right on the fly ....so you can gather a lot more data on each run without having to use the help of spanners.
A test mule should have a lot of adjustability on the fly methinks.