Well, that's a tricky one.
One thing you can be sure of is the new Futura101 won't leave you short changed in the actual (displacement) volume department ; you can be assured of a full 101Lts and enough change to settle any arguments about water density, temperature or even altitude.
Taking into consideration the extra overall and (significantly) tail width of the FU101, you will have a board that "rides" bigger than previous boards of similar volume, so combining the width and the full measure volume, the FU101 certainly rides closer to the borrowed ST115 than the size number alone might describe. If you're the type of rider that doesn't like too much board for a given set of conditions, the potential to take the "smaller" FU101 option for the higher wind days could be pretty attractive.
However, by(typical) sail size sweetspot range, running a 7.5m on the FU101 is pretty much pushing the bottom end of the practical (wind) range of the FU101, and as a medium+ rider weight, you'll be feeling that sooner than lighter riders in marginal conditions. There is no doubt that for an 80-85kg rider looking for well placed performance around 7.5m, then the FU111 is a more ideal Futura choice in those conditions.
Comparison in high wind riding depends a little on individual style; both the Futuras are fairly easy boards to ride "casually"; that's to say, they don't require or demand a specifically technical style nor a lot of rider input to get good performance from them; in many ways they actually can give great performance being left to bounce around on its' own rather than being sailed locked down too rigidly as would be the case (say) with an older board or a harder rail slalom etc. For some guys this is definitely an advantage, others are more sensitive to the "out of control" (or is it the "need to control" feel) and that factor alone is clearly worth one size difference 9at least) when it comes to the rough stuff.
I'd look long and hard at your expectations and sailing % time at the 10-15 end of the range. As Maximus already noted (good details !), you gotta park a compromise somewhere if you really want to cover 10-25 with one board- at least now you got some idea/s on how to apply the choice.
Let us know if you want more info.
Cheers ~ Ian