I use iS131, iS107, F101 + Kode 86. I am around 90kg and can get out on the Futura 101 in around 15 knots with a 7.0m Severne Reflex and am still out there in 25-30 knots with a 5.7m Severne Gator.
I rely greatly on fins to tune the range of all my boards - even the Futura.
For your weight, I would suggest the Futura 111. It will fit very nicely between your other two boards, and take your 7.8 down to 5.7 sails. By the time you are fully powered on the 5.7 you could change down to your Kode - you'll have plenty of 'cross over' in your equipment.
Hope this helps.