I have the iS 111, and the sweet spot for sails is the 7.5. I also love my 6.6 on the 111. I weigh 78kg. However, I don't like my 8.4 on the board unless the wind is steady, which isn't often. My 8.4 rig (Maui Sails TR3 and heavy HPL formula carbon boom) is not comfortable in marginal conditions, but a lighter rig (RAF no-cam sail and lighter boom) would make a difference.
A one meter jump in sail sizes (6.5 to 7.5) is very good - not too close to each other. However a two meter jump from 6.5 to 8.5 is pretty big. I think I would choose the 7.5 now and later go for the 8.5 if you get a bigger board for lighter winds.
If the wind is marginal, I usually choose my formula board with a 9.2 or 11.0 and don't use the 8.4. If the wind gets to about 14+ knots, I can then use my 7.6 on the iS.
Can't help you with the mast and sail situation other than it's difficult to know how a sail will perform with a short mast and long extension until you try it. Generally, the leach will loosen up a lot and the sail may fuller at the bottom (deeper draft). If you go with the 8.5, I wouldn't buy a mast until you try it with the 460 mast.
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.