OK, so all issues solved then? Great. As Chris indicates one thing that can be surprising at first is the boards tendency to keep speed and even accelerate in the (bottom) turn. To use this effect, you might wanna stay a bit more centered over the board (or forwards) rather than lean back.
As for the "water touch effect", that isn't really anything I experienced. And I use fairly long lines (30") and still run the boom comparatively low these days. But on the 76 and 81, you do stand quite far back on the board, so if you run your mast foot in the usual place, the boom will sort of appear lower since you lean the mast back more, I guess.
Sail choice. I've sailed the Quads both overpowered and underpowered, and I think they handle both well. So it is essentially up to you if you will use this wide range to go big or small with the sail. Personally I tend to "trim" the sail size a bit according to the conditions with smaler sails in better waves and bigger sails in onshore etc.