I've used my iS101 with anything from powerful freeslalom 7.5 to stable but not som powerful 6.6 speed sails as well as the rather flat and super soft 7.0 Hot Superfreak pure dacron sail. The speed, stability and general feel you get from the board depends on the sail, and to utilise it fully, a stable sails is the best. But even with the Superfreak, the board works and actually becomes almost like a freerider board. I even didn carving 360s and suc things on it which it not the first thing on your mind with a full on slalom setup.
I don't know the Severne sails in detail, but my advice is geting a type of sail that corresponds to the sailing you want to do. My guess is both 7.5 and 6.5 overdrive would work great. I'm 10 kilos lighter than you but for me 7.5 far from the max sail limit.