Reminds me of this thread a bit. The stuff CW posted there helped me understand things a bit better (although I'm still not sure I have it all right!)
The way I see it, 'more flow' can only refer to the turbo itself (and subsequent replaced pipework for FMIC etc). So the advantage once the air gets to the intake is that it's cooler. But it's not a lot denser - air density is relative to absolute zero (-273 deg C), so even a 30 deg decrease in temperate is only about 10% more dense. So I think that's part of the power increase, but that most of it comes from being able to advance the timing. Am I even close to 'getting it' yet?