there may not be a big difference in speed bet. 2300 adn 2800 but you can bet there is a hige difference in fuel burn.
as soon as you exceed hull speed (roughly 7 to 7.5kts on a 40ish foot boat), fuel burn increases much faster than speed. typically, just an extra knot of speed, may cost you 30% more fuel or more.
you either need to run full settled in the water, at or below hull speed, or fully on plane. halfway in between, you will burn as much fuel as if on place but go much smaller, throw a large wake and lug the engines.
also, trim tabs are often critical to get those stern heavy boats on plane, make sure they work