I do have alternator whine, but with the gains on my amp all the way down, I don't usually hear it. It kicks in with volume set to 25-30 and I usually listen between 14-20, but it's there, no doubt. I'd love to eliminate it completely. The stereo shop says that some alternator whine is normal with the Becker head unit but I'm not sure I believe them.

I was thinking of installing one of these, but it might well cost me 5x more to install than to buy, considering how hard it is to get to the Boxster's engine.

Have you tried just extending that #12 wire to one of the ground bolts at the bottom of the center console, to see if referencing them both to chassis ground would help? The factory ground may be weak.