Interesting points and thoughts.
I'm 95 kgs (+100 dressed up). On my S-type 115 I could quite easily carry a 4.7 wave sail in close to 30 knots of wind and have fun. Few 65 kg sailors would find that combo useable. The board would simply get airborne due to their less weight. Anyway they could be comfortably on 80 litre boards and 4.0 sails and still have more reserve buoyancy.
I once tried my old Carve 145 with a 10.4 race sail (recommended max 9.0) and compared to my 9.4 sail I gained absolutely nothing but poor balance. The big question is wether a 65 kg guy would find that combo more useable than I did. I see no reason why he should. Actually my experience is that the recommended ranges for most boards are quite precise. We can move outside them (I read kevin Prichard telling he had a good result once with a 10.0 sail on a 97 litre slalom board!!), but I really don't belive that we, big or small get something really good from it, though it may be possible.