If you're sure your engine is running the way it's supposed to, then I would look at the prop pitch and dia. as the next place to gain speed. I'm thinking that a 19 to a 21 pitched prop would be about right for 260 horses...depending on what you want your boat to do.
And in all cases, welcome to the site. Glad you found us.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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