The transmission creates the self-centering forces through the cables to the shifter. If the shifter/cables are free and not binding, then the problem is likely in the transmission.
Here is another post regarding same problem:
http://986forum.com/forums/general-discussions/32756-boxster-shifter-does-not-stop-neutral.html
Also, be sure that what you think is "non-centering" isn't simply a shifter worn to the extent that the shift lever flops around even though the transmission has pushed the shifter back to center. In other words, it is worth checking the shifter and cables extensively before you go to the point of pulling the transmission (you'd hate to do that only to find it
was a shifter problem).