Something else to consider, the LS3 makes close to 500HP with a cam change, and headers. The LS7 makes 550 (probably underrated)...Even if a dealer discounts an install and buys back the LS3, it's still not good bang for the buck even at $5000 premium after LS3 buy-back. Having said that, that LS7 is one seriously cool motor.
(edit) I take that back...that's NOT BAD bang for the buck
but it will void your drivetrain warrantee, like most engine performance part installs.
Maybe if someone like Tom Henry was willing to do a LS7 swap for about the same price as an add-on supercharger it might sell very well.