Si-drive remaps the throttle response and, with Cobb AP, changes peak boost as you've said. I don't remember the peak boost in I-mode. S is about 16.5psi peak, and S# is 19psi peak. I get spikes to slightly over 20 at times [Stage 2]
So I guess by going hybrid-BC, the Si-drive feature would have the same peak boost in all 3 modes but different throttle response? Not that I care about it anyways.