Hi Myes Myes, I'm a bit heavier at 76kg

I was riding the 8 LF S2 at Safety Bay t'day - check the wind graphs more than 10 knots range and was always completely comfortable (could have used my 12) (board 128 x 40).
I would recommend the 5th line, I converted all my kites 8, 12 & 17 the bar complete with lines is about 400 bucks. Recovery from a throw to leash takes about 40 seconds, I kite when I want and can easily self launch and self land (no knots, or pull) using the fifth.
I don't reckon it has naff to do with range of the kite - what you do with the board and kite will dictate the range to a large degree IMHO. The increased safety alone is worth it.
Just do it!