RE: S-type 115 or 104
While I don't have an S-Type 104, I do have a similarly sized slalom/freeride that's about a 100 liters and 61cm wide. It carries a 7.0 super easily, and I believe that it would also work well up to a 7.5. I'm not sure why Starboard limits the S-Type 104 at 6.5, especially since its smaller brother, the S-Type 93, has the same 6.5 upper limit. I could be because Super X type competitions would tend to favor smaller sail sizes to facilitate tricks and maneuvers, but frankly I'm guessing a bit.
As Willy has noted from experience, the S-Type 104 does work quite happily carrying bigger than recommended sails. Also, as you found out sailing a 6.7 sail on S-type 104, Starboard's upper limit recommendation appears to be understated. That's good for you, because the S-Type 104 is a better fit with your existing 89 and 125 liter boards. I think a 115 liter board is too close in size to your 125.
Still, it might be useful if someone on the Starboard team would comment further about the unusually low size limit.