Did you check the weight on the scale when she was last lifted ??.

I can 17+ with my 1979 43dc with 310 hp 671's at 2250 and is on plane, tabs full down. Now I run mostly at hull speed, too expensive fuel and wear to keep hoofing along on plane plus it gets noisy. I'm convinced that the factory undersized the exhaust, muffler people say it should be 8" not 6" as factory installed.