RE: Board/sail combo for gusty flat water sailing
I owned both an HS 105 and a 111. I can confirm that the 111 was overall superior and better from most points of view, but I also reckon that both showed the same kind of behaviour. The HS is actually extremely fast and efficient and probably one of the best possible boards to deal with wind holes (when planing); nevertheless, IMvvvHO it is not a good board for on/off conditions. Point is that the elaborated hull, while extremely efficient once one is going and planing, also shows a lot of drag in pre-planing conditions. This needs to be addressed either with specific planing technique, or with heavy pumping. In both cases one can easily get tired. My guess is that a board with a more traditional hull, with usual slight concaves and a flat(tish) bottom, would make things much easier. Probably an iS would be OK.
I would also suggest not to put an overly big fin under it. In fact big fins, while being more efficient going upwind in lighter winds, also add drag when starting to plane. Smaller fins may require to head for downwind a bit more when starting to plane, but will speed up quicker and ask less of one's resistance in continuated on/off conditions.