I have an '08 iS 111. I weigh 78kg. The perfect sail is my Maui Sails TR4 7.6. I use my 6.6 on occasion, but it is a pretty rough ride when the wind get over 20 knots. I occasionally use my TR 8.4 on it and when powered (12-15 knots). it is very comfortable. If you have steady winds, the 8.4 will work great. I only use one fin, a Tectonics Tomcat 42 which is great. A little small for the 8.4 and a little large for the 6.6but a good one fin compromise.
My next larger board is a formula 160, so if I need the 8.4, I usually choose the formula board and a 9.2. Where I sail it is very gusty and slogging on the iS 111 and 8.4 is not fun.
If you are highly skilled and race slalom, then the 7.6 and 8.4 would cover you best. If you are advanced and just freeriding, the the 7.6 and 6.6 would probably be better.
When the wind gets over 20 knots, I prefer to go to my 105 bump and jump board and a 6.0.
I hope this helps.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.