I'm a little bit lighter than you, 87kg.
If I have an average 16-19 knot and rather stable wind, the 7.8 works perfect, with a 40 cm fin.
If the wind is not stable enough, I use the 7.8 on my IS122 with a 43 fin.
The 6.7, I use typically in 18-22 conditions with a 34cm fin.
If you plan to use the 7.8 from 17+ knots, I wouldn't worry to much about sinking unless you have mega big lulls at your surf site.
IS87 / IS101 / IS122 / US147