If I recall correctly, the new booster won't even connect up to the old linkage, something doesn't add up here. I posted the tech bulletin here, but I haven't looked at it in about a year or so:
http://www.pelicanparts.com/techarticles/Boxster_Tech/57-BRAKES-Booster-ABS/TechBulletin.pdf
I'd take a look at it and then report back here on how it compares to your car.
-Wayne