I've never heard SPP alone without the Bose sub and surround speakers. However, the effects channel at the back is more of a gimmick at levels higher then the Sur_Low setting. IMO, the Sur_low option acts as an extended equalizer, putting less prevalent sound behind you while keeping the main track in front. Think of a jazz vocal performance - it would keep the jazz singer straight and center, and put any light instrument effects (cymbals, etc.) behind you. Some people don't like it, I do. Anything higher than the sur_low and the sound begins to distort and the inside of your car ends up sounding like a cave.
I would guess the Bose has the edge in bass response (when properly tuned ... for some sources, bass 0 isn't low enough) over the SPP as I don't believe the SPP has a separate sub for the lower frequencies.
The sub also doesn't belong behind you at shoulder level - should really be in the footwell but Porsche probably designed it this way to increase footroom in an already small (relative to sedans) cabin.

There are other options to sound, they would just much, much more. Probably starting around 3-4K. PM me for more info.