I'm not sure I followed every thing you said. Let me simplify and paraphase to see if I'm getting it:
I certainly agree that the boosted GN engine is going to "behave" differently than the LS1, and the designers no doubt had to account for that. BTW: I had a 1962 Olds Jetfire - also turbo charged so I remember that the biggest problem that boost causes is pre-detontation - aka knock.
While I never put 100 octane in the GN (didn't even know it existed till a few weeks ago), it was considered "a fact" in GN circles that the more octane, the more the spark could advance, and thus the more HP. So many GN owners used octane boosters.
I sort of assumed the same would be true in any car, particularly one with a computer running the show (as opposed to mechanical devices, like jets on a carb that might need to be adjusted, or a distributor that needed to be twisted).
When you used the word "unlimited" spark advance, then I can see where the designers wouldn't give the computer that far to wander. Probably enough to take care of normal variations, perhaps centering on the 92 octane?
So in a nutshell: If I were having pre-detonation/knock problems with 92, THEN switching to 100 would get a performance boost, but as long as I'm not, then the $4/gal isn't going to make the car any faster.