I decided to stray from the norm and go with an unusual tire setup. FYI, the 997TT wheels' specs are as follows:
F: 19x8.5 ET56
R: 19x11 ET51

With the 15mm spacers on all 4 corners, the wheels are fairly close to GT2 specs. I am running Michelin PS2s, tire sizes are 245/35/19 and 295/30/19. I felt that the 315/25/19 size that most people go with in the back is too wide for the 11" rim (if I had 12" that would be a different story) and is not enough sidewall for my liking. The 245s in the front are a better match (diameter-wise) than the 235s that most people have. The extra meat in the front should help reduce understeer. So far, no rubbing (not lowered) or weird errors...

 

Great car and nice taste in wheels. Allen at Wheel Enhancement suggested I get 5 mm spacers all around on mine. I went with the typical 35's in front and 315 in rear. No problems as of yet and have made many trips to the drag strip taken the pressure down to 25lb and currently running a consistant 12.5@115 on a all stock 996tts cab. Offsets looked killer at stock height but once I put h&r springs on the rears look quite tucked in. I don't like it. At some point I may look into 10-15mm spacers and of course longer bolts. Here are my before and after the lowering. Is it my imagination or am I right? They say white is the new black. Love the look!

 

Looks good. Love the 997 wheels on the 996. There are a lot of people out there that don't like it but I am not one of them.

 

It looks really good.

 

Frankly I think your red wheels look even hotter Your car was actually the one that inspired me to get these wheels. I wanted to do the same thing with white (actually, I was thinking of blacking out the face to make it REALLY stand out) but was worried about the masking job wouldn't come out as nice as OEM. Plus, my wheels were literally brand new so I decided to just leave them as-is for now.

Regarding the spacers.. I agree that the 15mm range is perfect for the rears but perhaps a little too much for the fronts. It's weird because with the spacers, the fronts are effectively 19x8.5 ET41. I'm staring at pictures of stock GT2s, which are 18x8.5 ET40 and look very flush with the fender. On mine, they look like they're sticking out a little too much. That or maybe I'm missing some negative camber...

 

Lower it then your wheels will flush to fenders.