Hey Marco, from what I'm lead to believe you will actually go slower on a bigger sail (even if you can manage to hold it down) than someone on a smaller one, due to drag (wind exiting the sail). Its not something I've noticed at my level of ability its just something I've heard.
I'm 100Kg and put the 8.6 away once it hits 20+ knots, for a few reasons
1. It hammers the sail/boom/mast (especially when its choppy)
2. Its bloody frustrating to waterstart big sails in high wind
3. You tend to muscle the sail and fatigue faster, I'd rather sail for longer and improve my technique on smaller kit
Definitely agree what speed helps manage a large sail, we tend to run larger sails on the broadwater because we often have inconsistent winds. If you were to sail somewhere like WA in say a solid 20 knots you'd probably be on a 7m or less
Still recon you should grab yourself a 7.8 and the right mast (CRITICAL) and you'll notice the difference straight away.