boat performance

[MattMigues]

so I'm double dipping I asked in boating but I'll aske here too. Ok so I have a new boat, used but new to me, I am running wide open 5100 RPM the boat is 15ft 54inch custom V 1/8" alum. Guy I bought from had 60 2 stroke Yamaha, I have 50 s stroke Yamaha running 10.5 x 14 prop. If I put my old jack plate I'm thinking I should pick up 200 RPM or so? Operating range is 4500-5500 but I know to get best performance and longevity it should run full RPM wide open. any of you guys can give me input? prop shop says I need to go down a pitch but says I'll lose 2-3mph so if I can pick motor up and gain the RPM I need I should get 2-3 MPH... Just guessing any info would be nice. GPS reading is 35.5MPH which is not bad but I want optimal performance.