i recall now why i didn't do the mod - cost. the standard 996 wheel well liner post -02 is vented:
Linings 02- For Wheel Housing
since they don't have different part #s for the c2 (narrow body) and c4 (wide body, similar to gt3, tt, gt2) then the standard 996 part should work fine. problem is, they are two piece liners, one at $136 and the other at $189; you are looking at $650 for both sides. better just a hole saw. ps, looking at the 996 liner, it vents two ways - into the wheel well as well as down.