FWIW: We have an '05 282 which we purchased with original low hour F225s with the standard SWS II 17P stainless props. Use the boat mainly in Pac NW out of Westport for salmon, halibut, tuna.
Yammies hit 45 mph tops in sheltered waters in light load (Puget Sound) testing. Got 1.4 down to 1.2 at best in real conditions. Always have full fuel, lots of water, ice and gear so never run light. We switched to 17P four blade props from Ken at PropGods and low speed planning and economy did improve some.
In 2018 we repowered to Suzuki DF300s and after adjusting mounting position (second from bottom) and props (Merc Eco Inertia 3 x 16 x 19P we are getting 50-52 mph tops (again lighter load in sheltered water) but here are the real important numbers: Typical halibut trip outbound running into the pacific swell, 3-4 people, loaded with lots of gear and ice at ~ 20-24 mph, the digital gages show around 1.60 mpg. The return trip usually hits in the 1.8 range. We've seen it hit both sides of 2.0 coming back in smooth water with a slight swell. Numbers slightly lower for tuna as ice and full live bait tank add a bunch more weight, so maybe in the 1.4 range heading out depending on water conditions and speed. Having to run slower into the swell really hurts economy.
In early testing on Puget Sound, we were easily able to plane and hit 32 mph on a single engine. Have not attempted that during typical use but there was no way the old F225s could do that falling off a cliff. Also, the lower fuel burn rate extends range (we'll sometimes go out ~75 miles) and/or enables a second day fishing without refueling which can help work around the fuel dock schedule.
Our gages use GPS to measure speed over the bottom, so relative to speed in water can be baised either way.
(We never shut the engines down, so they run/idle/troll all day). But we have consistently found that the predicted fuel use (the electronic fuel flow system embedded within the Suzuki controls) is extremely accurate. If the gages say we used ~103 gallons (long tuna trip) we likely take about 101. They gages are always close and always slightly overstate the actual use, so just the way we like it.
In any case, we're at least 20% better across the board with the new motors.