Thank-you very much for your help. Christine's posts are most interesting (I took French in school and lived in Québec for a year, but my skills are rusty).
I'm not at all interested in playing the "how low can you go" game. 7-8 knot winds require more rig than I care to work with.
Also, I gave the wrong impression, I wouldn't try to go 10.0 down to 6.3; 10.0 to 7.5 maybe, or 9.0 to 6.3 probably - I have an 8.0 and 6.3 in the SuperFreak and that jump is a piece of cake. So to me the gap kind of depends on the sail model. Further, I would not be likely to go 10.0 iSonic / Futura down to 8.0 K96, but would stay on the larger board until, nicely powered with smaller fin and 7.5-8.0 rig (maybe even the 6.3), and then go to the smaller board.
In the Futura, the 144 may be a better equal to the iSonic 122, because of the tail. I've thought about the 144 instead of the 133, but was edging toward the 133 for top end speed.
As to the C131, it was a sweet board, I admit. I really had no complaints, used it in 6.5 to 8.7 conditions, it jibed nicely, could handle small swells pretty well, reasonably smooth ride. My only disappointments were that it didn't plane quite as early as I'd hoped and it never really gave me a huge thrill. I'm a fan of easy jibing boards (have had easy and hard jibing boards, and see no reason to make it harder than it is...), but maybe the 131 made it TOO easy? It was a superb all-around board, but somehow it didn't have the spark to light my fire. Perhaps if I'd gotten it in wood...
The A117, in contrast, is slashy and maneuverable on swells in the large lakes I sail (Great Lakes, USA), workable for planing-type old-school maneuvers (carving 360s, Donkey jibes, etc.). Downside of the A117 is that, in my view, it really is best as a wave setup. It benefits immensely from having a little swell to start planing. I think those who have knocked on the A117 were trying in the wrong conditions, wrong setup, and perhaps wrong rig. It REALLY responds to the 8.0 SuperFreak. In my view the A117 is a keeper, though it is a somewhat specialty board. Am glad that SB kept up the bloodline in the Kombat.