I have an iS115 as my biggest board and sail it with sails up to 8.5.
Although I weight a bit more than you (83kg
) I can get it going in 10kts ish but I need an initial gust to get it started. Once planing it stay there quite easilly. However, in these winds I would be happier on a larger board such as the iS122 especially if I intended to race. The 115 can be hard work in light winds where a little extra volume would suit me in sub planing moments.