Help from Sceptre Owners

[schooner]

Hello,
I have just repowered my '85 Sceptre with a new 225 Johnson-Suzuki 4 stroke and am still having trouble dialing in the prop. Center consoles with equivalent power seem to be getting 10-20% more speed through the range. Should the extra weight of the deck and cuddy make that much difference? Currently swinging a 15.5x19 prop and getting statute 19 mph @ 3500 - slip=28%
23 mph @ 4000 - slip=24%
27 mph @ 4500 - slip = 20%

The motor is mounted as low as it will go and the anti-ventilation plate is about 2 inches above the lowest point of the vee at the transom. WOT is 5600. The above numbers are with about 100 gallons of fuel, 2 POB, and the canvas up.

Would be grateful to hear about the set-up for Sceptres with equivalent power and how performance compares. Wondering if what I am seeing is acceptable. MPG is around 2.5 across the range. Thanks very much.