For me, I had 2 issues:
The year of my 928, and cost factor....When I first got my 928, I had seriously thought about installing a SC, (I really wanted to do it!) but, for the $5K or 6K it would cost to do it, I could buy another car. LOL Instead of going the SC route, I bought a complete hi-flow exhaust from 928MS, which made a nice increase in power....Very noticeable gain, too.
And if you add a set of headers (along with the new exhaust) you probably have your 30-40HP total.... And you'll probably have spent less than 2K for everything.
I think if I had a newer model (maybe an S4?) I may have done it...I just couldn't see spending that much on my 1984 model.
Just my 2c, and personal preference, I guess...
Although, a SC is certainly the way to go if you're looking for a serious boost in power!
If memory serves me correctly, I think the Stage 1 SC will give you approximately a 30-40% gain in overall HP. (Maybe 70 - 80 HP gain) or thereabouts...And I think the Stage 2 SC should yield around 80 - 100HP gain...
Not too shabby!!!