Well as far as fuel economy is concerned you need to gps the distance against a know amount of fuel. If you do this and get between .6lbs (best to expect) and .7 lbs (worst to expect) per HP/hour you motor is running right. If it is higher then you need to have the motor checked. 1 under performing cylinder could affect up to 25% of the total HP. My GPS lets me set a threshold speed to include in track distance so I set it at 3.5 MPH to eliminate time on the trolling motor (boat moves 4.5MPH at idle speed). On my boat, not an action craft, I get the best distance out of a tank of fuel at WOT (about .3 mile per gallon better than 3/4 throttle). I measure this every time out.
I get an extra 1.2 miles gallon using non ethanol fuel. Also get an extra 1.4MPH.
I think you top speed is about 2-4 knots off where is should be. Unless your prop is beat up, I do not think that is your issue. The pitch should be stamped on the prop see if you can find that and let us know.
I noticed you had tabs, do you run with any bow down in it? This of course reduces speed and fuel economy.