will I lose any rpm's with a pre loader?
Not if your pump is loading properly now. If you do, you were not loading the pump.
will it hurt top end?
Almost the same answer. If it pulls the rpm down a little and slows the boat any. You were not properly loading the pump to start with. Trimming the impeller to regain your rpm and sometimes putting a little larger nozzle works to correct the top end deal.