I went up a couple months back and listened to the 997 Cab they were working on. It was magic.
What I'm getting done is what they call an "enhancement". They'll add an amp and a sub in the passenger footwell (this is a bit of a bummer--it's going to crowd that space by about four inches by the center console).
The amp works in conjunction with the stock amp, tweaking the output levels. I think he may add some resistors to the dash tweeters as well. The custom subwoofer, and its placement are key.
It doesn't sound like a lot of work, but it's seriously awesome. I only heard the 997 Cab in a setup that was about 75% complete and I was already sold. Imaging, sound stage, frequency response, etc.--they're all as they SHOULD have been when you ticked that (@#$^#$@ option box for the @(#&*$^@#^ Bose system.
Mine will be the first 987 they've done, after that they'll be able to offer a "kit" so to speak which should be able to be installed by them within a day. The price will probably be over $3000 though. Again, you're paying for the sound, not for a flashy, blinky head unit and two twelve-inch subs that rattle your door panels (although don't be fooled--that sub puts out plenty of bass).
If you love music, wait for my review and decide for yourself (or come listen--anyone in SoCal is welcome to come to SD and open their ears
). My car should be going in at the end of this month, to be done within a week or so.