If you've got a prop that lets the boat top out at the right RPM, what will happen after it's labed? Let's say you gain 2mph, will that new top speed occur at a higher RPM, or does it just reduce slip, thereby maintaining the RPM at a high speed?

Thanks

Depends on what is done to the prop. If its a standard lab, it will pick up rpms as well as speed in most cases.
You can add cupping, change rake and increase pitch to reduce the rpm increase.