RE: new vs. old slalom shapes
I am not sure that efficiency increase can compensate wetted surface increase. A short/wide tail will vary wetted surface greatly with trim variations, while a long/narrow one will vary comparatively less. As a matter of fact, boards built for top speeds are narrow, not wide. Plus, I am not perfectly sure about how reduction of tip effects can compensate increase of trailing edge effects; were we talking about a wing travelling through air, I would totally agree; but I confess my ignorance about what happens at the air/water separation surface when a board skims over it.
Of course I totally agree about wider boards being fast easier and more often.