Take a look at this thread regarding different MAFs. Being that your car is a 2000S, you might want to check on a couple of things:
http://986forum.com/forums/boxster-general-discussions/68392-need-help-air-fuel-mixture-problems.html
When I added the 74mm TB, I got a silicone elbow that fit the new TB and replaced all of the stock intake tubing with aftermarket tubing from AirRaid. I was able to get rid of the Heimholz chamber (intake muffler) and improve the intake flow along with insuring that there were no vacuum leaks.