> First the highly productive staff often make less under that model (where poor performers make more)

I just wanted to share the fact that there are places where tips are shared by the whole staff, and that system works well (I can't say if it works worse or better than the one you're describing as I don't have much experience with it). It also has the advantage that non-public-facing staff (e.g., cooks) get their share of tips too.