Nope - I have not had any trouble since I set it down deeper into the water. At the recommended 1/16" or whatever it is lower than the hull I would lose the bottom at speeds above 12 knots or so. I set it to a little better than 1/4" below the hull and it rarely loses the depth now. I don't think the need to lower it was caused by the tabs, but am not sure. The position of the tabs when running doesn't seem to have any effect on the transducer.
If you take the time to install a plastic mounting board on the transom and then screw the transducer bracket to it you will be able to experiment without adding extra holes to the boat. If I was mounting another one, it would be screwed on so that the highest point in the range of vertical adjustment was even with the hull bottom so that all the available adjustment was downward. I think they suggest you center the mounting screws in the slots with the transducer positioned at the recommended spot.
Interesting note - I also have a Raymarine depth sounder with a thru-hull transducer and I always have both it and the dual-freq Garmin on at the same time. They do not interfere with each other and are always within a foot of reading the same.
Oh, and you sure picked the right tabs! I had those on my CD22 and it was like they were designed for the hull. You will likely never have them fully down when running, but it is nice to know that the extra is there if needed. The Lencos on my CD25 are all the way down on one side or the other almost all the time, and they still do not quite do the job.
