Here is one guys 3.4L tuning solution that was featured in Excellence magazine. He is a regular street driver and not a shop owner or track rat.
986 Boxster story
There used to be tons of Dyno data for 2.5 Spec Boxsters regarding different exhausts, underdrive pulleys, intakes etc. but all of it was lost when Brad's site went down. As I recall there was concern for the cars running too lean with straight pipes and actually ran better with a muffler of some type. Too bad it's gone.
These cars are all running lean with a stock 2.5 showing the best tune and a steady AFR.
http://boxcar-racing.com/forum/index.php?topic=688.0