When I finally decide to get aftermarket wheels, I may get 18s for weight savings purposes. I would get 18x9 fronts and 18x10 rears, and run Continental ExtremeContact DW tires, 265/40R18 (26.3"tall) front and 295/35R18 (26.1" tall) rear. Of course I will have to make sure that the wheels are designed to clear the brakes. Using an ultra lightweight monoblock forged wheel, this combination would save over 30 lb overall from the stock wheels and Bridgestone tires, and have 40mm (!!!!) more width all around...talk about helping to kill understeer.
If anybody wanted to MAX out width, you could go with 19x9 front with 265/35R19s (26.3" tall) and 19x10.5 rear with 305/30R19s (26.2" tall). Rolling the rear fenders might be necessary, but DAMN, 305s would be great in the rear.