RE: q on isonic and sails
I second Screamer's view. It's mainly the water surface state relative to board and fin that puts a limit in higher winds; not the sail usually, as modern race sails can really take a lot. And at 78 you are relatively light, but not that much light after all.
If I was in your shoes, I'd rather try with smaller fins. 26-28, depending on construction, would tame the ride quite a bit and let you use your 5.6 more easily. Personally I (84 kg) find a Deb SL2 32 is just perfect on my S95 and 6.3, unless the water is very rough, then it's 30; so I guess you can go down one size for the sail size, and another one for the body weight.