Skidpad numbers would be one thing, but really this kind of thing can only be tested in a set course type of environment such as AutoX, racetrack lap times or (not that I'm recommending...) canyon runs. Each time I messed with my suspension I've gone to the two safest corners on Angeles Crest to see what kind of safe entry and exit speeds the car will muster, but even this is not good testing form as the weather changes all the time, the driver changes all the time, etc. But you do get numbers out of it, and with the right equipment you could even datalog for further and better analysis.
That said, nobody's going to do your testing for you unless you get lucky enough to have a predecessor. With my setup (BC coils, all soft damping, 8k/6k springs, 245/35/18 RE760) I will say I gained an easy +15mph safely on Crest, but again there are too many variables for even one car to keep consistent, it's nigh impossible for two different cars to match setups and performance.