It will affect your acceleration a little bit, but if you lower your car it will look amazing and I personally think that counter-balances any negatives of the 20's.
I have 20's shelby CS67's and when I put them on I noticed a difference...now I also have lowered the car and added an intake and tune and the car has no problem turning those rims...sometimes the ground needs to catch up to the tires...
The added mass of a larger and heavier wheel shows up more in suspension action and braking than it does in acceleration. On a street driven vehicle, it's probably not enough to notice, but if you were to go autocrossing or participate in a track day, then you would notice it if you could do back to back runs with the stock wheel/tire combo and the aftermarket wheel/tire combo.
Thanks all for the replies. 07 will be the year for my 05. My current mods include K&N CAI and Bassani axle backs. Besides the new wheels and tires I plan on installing Cervini's front facia kit and the SCT Livewire when it comes out. Maybe the Saleen SC later in the year.
Before any other mods were added I had the car re-Dyno's at the same shop.
With JLT Cold air and SCT Tune (Canned) I hit 285rwhp and 311rwtq. Once I put on the 20's running the same tune I hit 275rwhp and 290rwtq. So a loss of 10hp and 20tq to the wheels.
Stockers with tires weighed in at 45lbs each, Foose with tires came in at 60lbs each.
Added pullies (BBK), FRPP Deletes and a new tune from Bamachips and I am now hitting 295rwhp and 310rwtq. Hopefully yhe new aluminum DR will get me over 300rwhp.