Can't comment on the different sails but as a lightweight ( although heavier than you at 65kgs) maybe it's that the sails are designed for c 80kg guys so they are designed for best performance in a stronger wind strength than you use them in? e'g - I use a 7 cambered sail in 12- 18kts . A 80kg guy they would probably use this in say 20-25kts? maybe more.. so they are probably designed to work best in these conditions?
I'm sure the sails are optimised for heavier people and winds so they may not be give the feel they are designed for for lighter winds???

Just a thought...
I know I have a 115 freemove board ( potato chip 75cm wide). I use it in 5-15kt but I'd imagine they are designed for 15kts +.
I usually am on a smaller board in 15kts + but a few times when I've been caught out in 15kts + the board just seems to come to life! I think..hey so this is what it's designed for ?

What do people think am I barking up the wrong tree or is there something in this line of thought?
Ps - I started with wavesails when I was freeriding ( I used to wavesail so didn't know any different.) I've since switched to cam sails for speedsailing and when I go back to the wavesails I hate the on / off power.. I think a freeride sail would be much better especially if you tend to rig to be controllable in the gusts rather than plane in the lulls.