I have 10mm on front with 8" ET57 Turbo II wheels. You can def go with 12 or 15mm. Rennlister MYW has exactly same wheels with 15mm on front.

MYW's car:
https://rennlist.com/g/album/7296714

 

Usually the offsets for the 997 front wheels are pretty much the same for NB, WB including turbo so not sure what you're trying to match.

 

In my opinion, an 8.5" wheel with an et 50 is the optimal 997 front setup or whatever width and spacer is the equivalent if running a narrower or wider wheel. Anything further out is beyond the fender line

Here's what that looks like



I had some 8.5 et 42 wheels on the car at one time and they looked bad and negariveley effected the handling, making the car darty at high speed instead of dead straight and stable - same alignment, same pressure, same tires.

Those looked like this

 

 

do you have the real turbo II (8.5 inch wheels +56) or the turbo II wheels that came as an option on the c4s (8 inch +57)?

if you have the 8 inch (c4s option wheels) leave the rears as is and you can max out 15mm spacers for the front. do 12mm if you want to play it safe... 7mm is too mild imo.

with that said keep in mind re petza's findings as while the wheel face may be flush to the fender, the new wider tire designs with protective tire lip will protrude beyond the fender.

if you have the 8.5 inch fronts (off of a turbo). you can do 5-7mm spacer in the front. 7mm if you want to get flush to the rears, but 5mm to be safe.

there was actually steal recently on ebay re oem turbo II wheels (from a genuine turbo rare 8.5inch front) for 650 shipped. they had rashes but would have been fantastic for those who want to do a color change (ie. to have the oem black wheel option which looks stellar).