I shall explain: If you increase your rim size, such as 20's on the rear, and keep the tire profile number the same, you will end up with a larger rolling diameter, you then would positively want to lower the car, as it would have a jacked up look, but then the problems with this set up are over-gearing and more rotational mass, that is somewhat of a drag. If you keep the rolling size the same, by lowering the profile number, on your new 20's then you will have that skinnie look to the tire, and as you know the tire is borderline of that now. The tire to wheel balance is now very good, and leaves little room for improvement. And speaking of lowering, you gotta lower the front. I have done it on both of my SRT's and it looks great. The car is to high in the front. I'm talking about one half to three quarters of an inch, no problems with the front spoiler at all.
As for lowering the car, please take my advice and buy the Aldan billet coilovers, three way adj shocks. I can get you these for a very good price. The labor to remove the rear shocks is very much more involved than the former GTS, as you have to remove the A-arm. Another advantage to the Aldan is that you have much adjustability. The ride hight is very adjustable. The fronts are a must, and they are very easy to put on.
As a final note, please don't over-gear these cars anymore than they already are.You will end up buying a clutch, as the one in the car is not to great as it is. They really look pretty good with the 19's on the rear, and the rim looks good to. Chrome the rims, that would look great, as the clear coated polished wheels will look worse by the day. Spend your mod dollars wisely.
We have been blessed with 19's and a good looking wheel/tire, and the aftermarket wheels are so very high as you know. The front should stay as is also, in my opinion.
Skip White