I have always had the best luck turning higher RPMS.. the motor runs better and there is just not that much stress on the system.. Also if you talk to some prop guys, you may find that when you downsize the prop, spin it a little faster you may go faster depending on what the boat needs.. You may need some bow lift ,transom lift, slip or less slip.. Those things can be labbed into the prop..
good luck
I know on my OB... we tried 5-6 props before we decided on one for the Parker 300.
If you talk to a prop guy.. they will usually let you try different ones before purchase.
good luck