I have a 1648 alumacraft jon boat with a 20hp 2 stroke mercury on it and I run the same as you,maybe a hair deeper even, like 13/4 -2" below the bottom of the boat and while it may not be optimum height my boat screams with over 1000lbs of weight in it, 28-30mph. I'm sure there is some drag there because I know my anti ventilation plate is buried for sure. I just don't think moving my motor up will run any faster but maybe run a bit easier which would raise fuel economy a ton I bet. I'm going to fiddle around with mine and raise it as far as I can go also without feeling nervous it may pop off,I'm going to experiment with it to see how much the performance goes up because I'm a big guy at 300lbs so I know the back of my boat is really buried when running so I can probably move mine up a ton and still be under the water. I'm curious to see if u do raise it how it performs. Someone told me to leave it alone if it performs well as it does but if I can gain a hair bit performance and save on gas,why not. I may make like a transom extender out of aluminum so at least the clamps have something to bite to then when I find that perfect running height, bolt her down permanently. That is something you could do to your boat,just extend the transom up a few inches and problem solved once u find optimal height,or just leave it be if it runs great,the worst case is you have some extra drag like me which won't hurt anything from what I been told. I can't complain getting 30mph,any faster in a tiller boat is scary anyways,lol.