[Accelo]

"Said their testing on similarly equipped LS3 with Fast Intake netted around 570hp/530 TQ. Said the LS1 should be very close to that."

The 6.2L / 376 cu. in and the LS1 350 cu.

That is 26 cu difference. Equates to 26hp difference everything else being equal. (1hp per cu inch)
That's a 4.5% difference. I guess it depends on what you call "very close"?

In your example the motor is making 1.5hp per cu. That would be extra 39hp attributed to the larger displacement.
Or almost 7% more hp.

An interesting fact is the new Corvette Z06 makes 2hp per cu inch. It does rev higher to get to this figure.