RE: 2 Boards for Slalom
It's a bit puzzling to hear your experience with the iS105 / RS6 7.8m combo, as mid 7m's in powered 15-20 kts are really quite a sweetspot for me with that board (in fact, powered 7.6m SSR/7.7CodeRed are close to my ideal sails on iS105). even on testing other (not production) iS protos like 95 and 90, that sail size was OK (good ok)even into winds well above 20kts and in rough chop..
(actually pretty funny ;a friend with iS105 asked me the other day "what's your favorite size for iS105?" - Answer : 7.6m. Then he asked " what about iS125??" Answer : 7.6m !!)
Did you have a chance to try the 7.2m (or some other model of ~7.6m) with your iS105, that would have been a very good comparison - and also a reference in case your RS6 or tuning is causing excessive loading onto the iS105 as the sail overpowers into the 20 kt range ??( Same trim/tuning would not be so obvious overload on iS125)
Also another good test would be iS105/6.7m RS6 vs iS115/7.2m RS6.
Especially in well powered conditions, it might be interesting.
As .g.e.o. commented, the iS94 (Sonic 95) is also very ideal with 7.6/7.8 range, despite it being a smaller and narrower board (and thus less suitable to being sailed overpowered with big sails than iS105).
You definitely should be able to get good range and coverage with only two boards - and iS105 + iS125 would have to be almost the dream 2 board combo for covering the 15-20 kt range.
Anyway, let us know and we will get it sorted.
Cheers ~ Ian