The "shag carpet" can cause significant drag, but I would not think more than a few MPH worth. Besides, it still does not explain why you are seeing virtually the same speeds as your old 330.

Used to own a 91 with a 365 hp 454 with silent choice that would run over 52 and had over 750 hours on it . 23 pitch prop is what I ran . The biggest problem with these boats is air or should I say a lack of , to the engine . I could get 2 mph on the GPS just by lifting the hatch a few inch's while I was running . I'm sure you would need more air than my stocker did . Might be worth a try after you get the bottom clean.

Well actually with the old motor the bottom was clean, slick for the 1st season. Previous owner kept it trailered. I could hit 48 gps. Then it sat in the water all summer 08 and winter of 2008/09. spring 09 it felt slower, could only hit about 42 mph. That combined with low oil psi, I assumed a rebuild was around the corner, so I took it slow all summer 09, added some lucas oil, and just cruised around, never went above 3000 rpm. Pulled boat out motor in fall, rebuild, 489 roller motor. Put back in water Spring 2010, bought jet skis for my kids, only put 14 hrs on new motor. Pulled boat out last fall, pressure washed scum off. I need to get the crap off of it anyway, so its getting an acid wash this week. What about the trim tabs?? Should I get rid of them.

update!! Had the bottom acid washed and buffed...56 mph on the top end @ 5200 rpm!!! The white chalky lime deposits were really slowing the thing down. This is with the 20p 3 blade turbo prop. Still need to try the 23p mirage. might try a 4blade 21p as well.