Ben, those numbers seem a bit low. I have a 20' Seafari, with a remanufactured Merc 150, probably putting out about 190-200 hp. I also have SlicKote bottom paint. With a new, stock lower unit, I can run about 42 mph on the GPS at WOT, loaded with 4 adults, full fuel, and the Bimini up. With the top down, just myself, and full fuel I can run about 47-48.
With my old lower unit, Bob's nose-cone w/ low water pick-up I could touch 53 mph under the right weather conditions: cold air, high humidity, 10-15 knot winds, light to moderate wind chop and running downwind. At 50+ mph, a Seafari positively
dances across the chop like a ballerina
It feels like it's almost airborne, just touching the tops of the waves; very light and nimble, like it
could chine-walk, but simply has too much class and manners to behave in such an unseemly fashion.
It's truly exhilarating. Just one of the reasons I love the Seafari.
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury