Are you reporting speed based on GPS, or the speedometer?
A ‘97 wellcraft with a 5.7 base engine (doesn’t matter if Mercruiser or VP) would be a vortec series 5.7 w/roller cam. You can rev them right up to their maximum 4800rpm without worrying that it will blow up. FWIW, if it does blow up, the engine was going to fail anyway.
i would run it again with the 15p prop. As I said last year, when wellcraft tested the boat, they used a 15p with an engine that made an extra 50hp. I didn’t see whether you used GPS to determine your speed, but if you’re not using it, you need to start. Boat speedometers tend to be less accurate than boat tachs. Lastly, on a boat that is underpowered proper trim is essential. If your trim is way off, you can lose a lot of performance. Also read the owner’s manual for Bennett trim tabs. Many people mistakenly think that the tab operation correlated to tab switch position. In other words, they think that pressing the port side switch up, means the port side tab comes up. In reality it pushes the starboard tab down.
Let us know now what happens.