Yes, the square root of the $$ means the rule of dimishing marginal returns is in play. Or in layman's terms each additional $ gets you less horsepower than the previous $$
The 1800 SR3 4.3 has two versions - carbureted @ 190hp - MPI @ 220hp. I have the 190hp version and it is fine for watersports (21" prop - be careful not to over-rev) and cruising (23" prop - top end in low 50's). I imagine the 220hp really flys. Not sure that adding MPI after the fact is in-expensive, easy or even recommended. But I'm an accountant not a mechanic.
If money was no object, you could just buy the more powerful engine and swap it - talk about diminishing marginal returns!
The accountant would start with a tune-up and a couple of propeller options.