One reason is that the "C" engine is balanced to the transmission, intake, and exhaust. To get more power, all really need to be changed. Personally, I have never cared for forced induction if for nothing else than it requires premium fuel instead of regular.
Do think a modern SIII 205 hp 3800 with a 4T65E trans would be the best swap and provides a number of improvements over the "C" or even an "L" but would be a major harness change.
Looks like it is on the way out though, is no longer a featured engine on http://www.gm.com/automotive/gmpowertrain/