Techinspector: I am re-looking at that option. Earlier inquiries elicited negative opinions on the subject. I just went, amonst others, to Whipple's sight. Theirs seems pretty impressive, with controllable boost and electronic management. They also claim the best low end boost in the business with the twin screw compressor. Unlike other units, the whipple provides very modest boost when the engine is unloaded then comes up quick when the demand is made. Boost is not rpm dependant.

Questions I would ask them would be:
What is the effective compression ratio
What is the longevity? of the compressor
Is it rebuildable if nec.

I've found a wide variety of prices. This link is for the best price I've found.

http://www.superchargers4less.com/whipple/gm454.htm