I think they're essentially the same, except for compression ratios. The GT has a bit more power low-end, but the GTS makes up for this by blasting the GT away in the top-end (lift).
So, if you reinforced both engines to the same degree and ran the same amount of boost, I would think that the GTS engine would still win.
And the reason for not seeing a GTS putting down 380+, is probably because more reinforcements are required in order to run that much pressure. But I think if a GT and a GTS are both running the same PSI under the same compression, the GTS would have the edge (due to lift), though this edge would become less and less pronounced the more power that's being put down... Eventually, the 40 HP boost from lift would be trivial, and they'd probably be about equal.
This is based on my knowledge of the two engines, and I definately don't know a lot about cars running turbo setups!
(Or a whole lot about cars, period