Does anyone even make a true CAI for the MKIII? I've never seen one. I've got 3" piping running from an air filter under my passenger headlight to the TB. Like Supra2NR said, you're not going to see any real gain in performance.
The biggest problem with the N/A's intake is the AFM, not the air filter or where it's located. The stock air box does a better job supplying the engine with cold air than a those on a lot of newer cars. If you're planning on just throwing on an adapter and a conical filter and leaving it in the engine bay you're more likely to loose power. This is because instead of improving things you've left the restrictions in place and now you've made sure the engine will receive plenty of nice hot air.
EDIT: I tried to vote but Apexi seems to have been left out.
