I had a similar experience. My 1994 290 Searay would go about 12mph
at 2000rpm and slowly ramp up to about 16mph at 2900, then stay on plane
at 27mph and 3200 rpm.

That's not all that similar. You were using
1200 rpm to go from 12mph to 27......not 500 rpm to go from 9mph to 25.