Given the fact that numerous types of exhaust systems have been installed on Boxsters with no CELs, I'd say your mechanic is probably wrong.
The codes you pulled can be caused by a bad MAF or a simple vacuum leak. What did yuor mechanic do to rule out the MAF or a vacuum leak as the culprit? I would strongly suggest, before you spend any more money throwing parts at this thing, you put the top in the service position:
http://986forum.com/forums/showthread.php?t=11272
....and inspect the oil seperator connections, vacuum hoses, and the intake manifold plumbing. There's a bellows at the base of the oil seperator that often tears, causing codes similar to what you have.
The bellows:
If you don't find any defects, you should try replacing the MAF. Make sure you get the exact same part number you had before, otherwise you may require ECM reprogramming.
Lastly, you might take Bisi up on his offer.