Here's my $0.02 on some of the issues as a proud GXP owner:
1) The LS4 cars are still relatively new and there was a limited production run with not a lot of parts common with other vehicles. That makes modding them expensive. I would think when they get cheaper more parts will come. Parts are still being designed for Ford Flatheads.
2) The big advantages of a GXP are that it handles great and that it sounds great IMO, not that you get a ton more power. In a straight line, I don't notice that much difference between this car and my old Buick Ultra with an overdriven SC.
3) The GXP and all other GPs have almost nothing in common electrically. Guys that swap LS4s into Fiero's generally use the entire GXP harness and all it's controllers.
4) The GXP has a one off 4T65. It's bellhousing is common with other LS4's (starter mounts to the tranny) and it has the same valve body as the Comp 1 tapshifts do. There are only 14,000 cores in the world (for the 14,000 cars built...).
Like everyone else said, as far as E.T. per $ goes, forget it. Stick with the 3800, it's not even close. Boost is the replacement for displacement. From another angle, if E.T. is your only goal, you should really go with something rear-drive anyway, because you can't beat the physics. IMO, if you really want an LS4 buy a GXP and recognize that you already have $1000 in tires and $1000 in brakes already on the car, with Bilstein shocks and a road-course tuned suspension.