RE: Pros and cons of smaller sails on Formula
I think the 10.0 freerace sail and 11.0 race sail will have similar range and planing performance, and either would be a good step up in lightwind performance from your 8.5.
Personally, I would go with the 10m freerace sail, because you are absolutely right about the durability and shorter mast and boom. Also, there's much less rigging hassle, and a slightly softer, friendlier feel to a freerace sail, at least in the lower and middle part of its wind range.
Of course like you say, hopefully you can try before you buy.