GT2 wheel set could be the perfect solution. You should be aware though that Porsche does NOT approve the GT2 wheels/tires for the 996 Turbo/C4S because of possible problems with the visco clutch/4WD.
So it actually doesn't matter if you go for aftermarket wheels or the GT2 wheel set, both are not approved.
Some Tuners offer 19'' wheel sets with 235 rubber in front and 305 on the rear. Might be a solution too.

Personally I'd recommend keeping the original stuff or going for the GT2 wheel set.