RE: S-type 104 vs 115
I am 71 kg, 180cm and I have sailed both boards. I have sailed the 104 with a Nitro4 race sail, but it is at the upper limit for the board, and it just did not feel like the sail fit the board. Sluggish reactions and much harder to gybe. 104 sweet spot is between 6.0 and 5.5 for my weight. My best day ever on that board was on a MauiSails 5.8 Global. I don't think you should even consider it for an 8.2 sail.
The 115 does carry a larger sail, but my opinion is that you would not be at all happy with an 8.2 on that board. I have found no problems with either board in choppy conditions in 18-20 knots, so that should not be an issue for you.
As Ray has said, if you want to use the 8.2, you should look at a larger S-Type or even an iSonic which can carry a much larger sail for the same size(115).