You should be getting in the mid to high 20's with twin Honda 40's. The wide open throttle should be in the 5,500 to 6,000 RPM (very minimum would be 5,000 RPM.
With a single of the 40's, you may not be able to plane, depending on the weight in the boat, condition of the bottom etc.
It does take some trimming to get the boats to WOT at max RPM--start up with the bow trimmed down, and then as RPM goes up, and the boat is on a plane, trim the engine back, and up--until MAX RPM, without cavitation or proposing.
There could be a number of issues limiting the engines, from bad spark plugs to carburetors which need cleaning--you should have the engine checked by a good mechanic before giving up on the boat.