I took my (new to me) 72 Eliminator Bubbledeck out on the smooth lake last week. I am running a stock, 250Hp Mercruiser 5.7 carb (4400-4800 WOT) motor pushing a Berkeley intake, Bayjet pump with droop snoot, diverter and ride plate. I was getting only 4200 RPM. The RPM and MPH were pretty much equal, 2500rpm = 25mph (GPS), 3500rpm = 35mph, 4200rpm = 42mph....It would not get any higher than that. I don't know what the impeller is. Does this seem correct or can I get more RPMs and speed from a better set up on my impeller?

Previous motor in the boat was an Olds 455 and the jet was rebuilt since then but may have the same jet innards from before

Thanks in advance for any info