Saw this linked elsewhere. It is a comparison, using an IS300 of the effect of aftermarket suspension stuff. Results are both objective and subject measurements.
The test an IS 300 stock, then w/ ProKits and Koni Yellows on full soft, and then the same on full hard. The test involves some type of autox-like track. The difference isn't HUGE, but keep in mind that is the most basic addition you can make and it is on a very controlled, low speed track. I wish there were more good tests like this for go-fast goodies.
Comparo