With the power your looking at putting out, you'll need to replace the stock internals anyway, so I would imagine that you could make any engine as powerful as you wanted. I think 1500 hp is about as much as you could expect from either engine. I think that I remember reading somewhere that at around 1400-1500 hp, the heads on an LSx series engine actually start to lift off the block because the head isn't held on with enough clamping force to withstand that much power.

Another question, do you expect to send 600+ hp through the front wheels?

Well, it is true that you'd need to lower the compression ratio to use any kind of forced induction set-up on an LS7, but I don't imagine how you could manage to crack the clynder walls. They're not that thin. Lingenfelter can get 800 hp out of an LS7, I don't know why you shouldn't be able to do the same.