I think the NA has a higher diff gear than the SRT, and last year I ran CLK55 18 / 9.5 wheels with 255 A6's in the rear. I generated a little more quickness off the line, but ran out of gear really quick. In most Autocross events, I usually got into second gear and stayed there. Last year, I got into third almost every race. When I was using my G-Tech RR computer a lot, I concluded that it cost me about .2 seconds to shift gears (up or down). Going to 18" wheels in the rear (or a higher diff) may mean more shifting in a tight autocross course. Gotta live by the data though.