For lightwind slalomracing the 7.5 will be most fun to ride, but the iS111 + 44cm fin should handle 8.5 too to glide a bit earlier.
Just make the decision:
Choosing 7.5 ==> using this sail from zero till its sweetspot (18kn average wind?) After this continue sailing with the 6.5 directly in its sweetspot. Making with 2 sails a rather big sweetspot
Choosing 8.5 ==> using this sail from zero till its sweetspot (15kn average wind?) After this you will have a small gap in performance but when it is good enough for 6.5 you can rock on the water again.
My preference would be the 7.5 solution, the sweetspot for iS111. Choosing sails at the waterside will be easy, lowwind= 7.5 and highwind= 6.5. When I had to choose for downwind slalom or figure 8 slalom it would be 7.5... Best fin in this case would be around 40cm, but I used also 44cm for 7.0 in lightwinds too boost overtaking speed
and my weight was +- 80kg