So how much does releasing a console first in a generation relate to sales?

[IIIIITHE1IIIII]

Releasing first gives you nothing but a very small head-start which probably won't affect the end result in any notable way anyway. It all comes down to collecting a strong user base and make them stay with the system like the PS1/PS2 did perfectly (unlike the Wii who ultimately let go, (that can still pay off in the end though)).