Besides the sidewall causing a "stiffer" ride feel, 20's on an auto R/T offer no performance gains. You still only get a 3.06 rear gear on the auto, not other gear choices are even optional from the factory.

On a 6spd, however, 20's with the Trak Pak net you an increase from 3.72 gears (standard on the 6 spd cars) to 3.92 gears as well as a limited slip diff (not an option on any auto or on the 6 spd with 18's) and the traction control off button will really turn the traction control all the way off. The traction control on any auto or 6spd with 18's will still be on slightly even in the off postion, probably to overcome the fact that you can't get a limited slip in these other configurations.

The 20's are also a tad wider, for possibly a little more grip.

I was thinking 18s and an auto, just as a second car my wife could drive and to keep initial and maintanence costs down (read-tires!). But after I found out about the gears, limited slip and traction control, I talked her into the 6spd (I already know she can drive stick, she had a 94 Mustgang GT with 5 spd manual) and 20's with the trak-pak. Now it is just a waiting game to find one on a lot so we don't have to special order one.