I wouldn't mess with the original sway bar unless you can find a way to shorten up the side of the sway bar where it mounts. I believe H&R or someone makes a sway bar for our cars or the SLK which will work. I don't think you can cure understeer with a larger rear contact patch. Understeer is somethng just about all cars have designed into them for saftey. I think Woody got it right buy going bigger in the front and smaller in the rear from stock. A stiifer rear sway bar might be an even better way to go.