I would have to disagree with a blanket statement that a "level plane" will give you the best speed/gph. I did many fuel runs with my previous 380DA equipped with smartcraft for instaneous GPH, a WAAS enabled GPS for speed and a buddy on board to record and calculate MPG (the most important number if you are talking about efficiency). On a very calm early morning time of day, before winds and boat traffic, on a local River at close to slack tide I did numerous runs to find my sweet spot in ideal conditions. On the 380DA, with no tabs (fully retracted) the boat would run with a bow up attitude. Nothing extravagant, and very managebale, but bow up none the less. If I applied any tab down, which would bring the bow down and "level" the vessels fore/aft attitude, my RPMs would decrease, and the MPH would decrease; hence, the MPG would also decrease. So, what I am saying is, that a level attitude doesn't automatically give you better efficiency.
Just to clarify, I did these runs with the same dillegence that was instilled in me by my Chemistry professor's.....accurate measurements give accurate results.....boy does that saying give me nightmares.....but wait a minute....we were talking about seat hieght....were'nt we....[:-boggled]