Hi, Mike.
I used a Fawcett extension for about 2-3 years before having it removed. On my bass, the low notes were really there, and I loved them. My experience:
-I really liked the E stop release mechanism, I could quickly and easily release and lock the E with my thumb, while in the middle of playing.
-It buzzed and rattled, especially pizz, and I had to constantly be tightening up screws and the like.
-There was one screw, attached to the main E stop, which stuck out pretty far (with a spring attached), and I was always worried that I would put my eye out someday.
-The greatest problem was that when the E was locked down, the E key also went down, and was in the way if I wanted to play low F# on the E string. That was a *real* drag.
-The rubbery material that held down the string got a good sound arco, but not pizz. Not really holding it down as solidly as my fingers on a fingerboard. And the rubber thing on the E stop eventually wore down/got compressed, and wouldn't stop the note anymore. So I rotated it, which solved the problem temporarily, but what would I have done when that side wore out?
-Also, on mine, the person who installed it did not make the nut area adjustable, so the low C was always a little out of tune. Nothing to do with the mechanical part of the extension itself, but important nonetheless.
Last but not least, when I had it removed the sound of my bass evened out and improved, overall. The person who took care of my bass during those years, Lou DiLeone, showed me a couple of messed up scrolls that had been twisted by C-extensions, and said if I didn't need one, I was probably just as well off without it.
I also tried out friend's Horst extensions, and really didn't like them. The keys felt less natural in my hand, and I couldn't really get used to the backwards thing (although the other guys said they did with time..)
I think the main advantage of the mechanical over the fingered is that you can move faster, if the notes down there will speak fast enough on a given bass.
Good luck with your project.
Brent