Flatty,
In stock form, (in my opinion) the 4200 kills a 4.8 when it comes to horsepower and the nice long flat torque curve. The 06-09 engines have been known to turn close to 350 horsepower with a tune and free flowing exhaust.
I will agree, with the 4.8 being of the LS family, there is a ton of aftermarket out there. The 4.2 not so much. Since the 4200 is a distant cousing to the Ecotec, there actually are a few sources for camshaft blanks and other hardcore components. Porting the heads on the 4200 really open up it's horsepower potential, especially on the exhaust side.
Adding a turbo, let's just say 450-500 horsepower is well within reach with the Inliner. Just search the project/build section here.
http://www.vortec4200.com/forum1/
The biggest reason I'm doing mine is because it's different. When I was younger, it seemed every project out there was a 350/350 combo. That turned into a 350/700R4 combo. Now it seems the LS is the mainstay. I just want something that is different than everyone else.
Chris