On the transom note, I played around a bit with the height of engine on the back of my boat earlier this year to see if it was possible to gain an extra knot out of the top end speed..... in actual fact, it didn't gain me anything other than cavitation.
What I found was that when the cavitation plate was exactly level with the keel of the boat, you could feel the prop loosing bite on the water when putting large amounts of power on from a standing start. When up on the plane, I could not lift the engine above the mid-way trim point without getting serious cavitation which was very much counter productive on the speed side. I also found that in a bit of chop, the prop would loose bite, and I'd have to drop the revs down for it to regain proper propulsion once more.
As it is now, the cavitation plate is about 1cm below the keel of the boat, which seems to provide the best of all worlds for me. Apart from when trimmed right out, the boat will go into a hard corner quite happily and come out without any hint of loosing bite on the water. I also now have full range of trim, and when trimmed out can get the best speed on the boat.
Your best bet may be to lower the engine by one notch, and see if that makes any difference, either better or for worse - if it's worse, try lifting it to one notch above what it was before.
Unless anyone on the forum will say for definate if it's something else, it could just be a trial and error situation I'm afraid