I had them on a 4m searider with a merc classic 50 years ago. IMO all that did was reduce the potential cavitation when lobbing the boat into a series of fast switch back turns. My engine was a manual start with no PTT. However, I also had them on a fletcher arroflyte with a merc 75 with PTT and they did left the stern up as slower speeds (was tail heavy at best) and made a massive difference once up on the plane with fine trimming adjustments.
As for fuel? the jury is still out!
These seem to work better in certain aplications than others.