My 2010 20’ Bennington SLi has a 2010 50 hp Mercury Big Foot 4-stroke. I can only get 4,800 rpm and a max of 15 mph out of it. The prop is what is speced for it. An independent marine shop serviced the outboard and tested it on the water. $3,000 later they represent that I should not expect more power or speed out of this combination (to be fair they also replaced injectors, pumps, plugs, wires, etc.). This SLi model was only designed for a max of a 60 hp motor. Does the rpm and speed sound right? The motor is designed for WOT of 6,000 rpm. Thank you for any insight you may have.