Is this possible? The RS wheels are 1/2" wider than the Turbo S at F9" and R12". Is this advisable?

Would the Turbo S tyres fit? They are 8.5x19in, 235/35ZR19, 11.5x19in, 305/35ZR19.

 

I think the answer is they fit. The regular GT3 has turbo size tires on GT3RS width rims, 9" and 12"

I just got the sport classic wheels on my 997.1tt today. they look boss, will have to get some pics posted.

 

The wider front wheels may be a problem. Porsche widened the front fenders and uses a +47mm front offset on the RS. This is a very aggressive offset with the Turbo S.
The wheels will fit but you will need to be careful with your choice of tire, especially if the car is lowered.

 

Don't both the GT3 and GT3 RS run 8.5" front wheels?

 

No. The new GT3RS has a 9 inch wide front wheel.