Since I've been seeing boats, answering posts, and talking to NUMEROUS folks that have been having difficulties achieving good performance, I'm going to take a few moments to share the top few reasons we are seeing:
1. Demand Linear not achieving 100% Demand (hotfoot and throttle cable improperly set-up)
2. WEIGHT where it's not supposed to be, NO WEIGHT where it IS supposed to be. SERIOUSLY!!! If you mount an Ultrex, (3) graphs, and custom brackets for 3 said graphs on the BOW, you should NOT be surprised when your boat does not perform. You've literally tacked a couple hundred pounds on the BOW of the boat. Then... some of the same boats swap out 3-5 Lead-Acid (or AGM) batteries to Lithiums, often pulling as much (or more) weight from the stern of the boat. This causes the stern to ride HIGHER, the bow to ride LOWER, resulting in more boat in contact with the water at top speed. No reasonable amount of trim will ever compensate for this.
3. Excessive Setback (especially when combined with #2)
4. Over-propped
5. Dragging Power Poles
While there may occasionally be an actual engine problem, it is completely UNREALISTIC to expect an engine to carry the loads/weight-distribution that is described in #2 and perform even reasonably well.
*** Simple Testing Hint: If you suspect your boat may be victim of #2, remove as much gear as possible from the bow. Add 80-120# of SAND (double-bag the sand) and place it on top of the back deck, at the very back. Go run the boat and try different trim/jackplate heights... if you see IMPROVEMENT (at all) in performance, you are on the right track.