Im a little lighter than you, and don't do unhooked (yet) but here's my impression af the 7m.
This season i had a two kite quiver, EVO 10 and 7m (2011). im 73kg.
The kites got me a lot of days on the water, but had a few days were the 7 was a little too small. and the 10 was a little too big. (on the middle setting on the lines)
Im out with the 7m until 16-17m/sek (32-33 knots) with lots of fun
imo. I think you will have a better quiver with the 8m. since your a little heavier, and don't have a lot of 30+ days.