My experience may be a little different than yours as my Cummins 6BT 5.9 (no "A") engines are rated at 210 HP. I prefer to cruise @ 1800 RPM and manage 8.5 to 9 knots burning a total of 10 gals. per hour. If I push them up to 2000 RPM I only gain 1 knot and the burn rate goes up significantly. So, I prefer to enjoy the view a little longer on trips.
1990 43' Marine Trader Tradewinds