I don't know specifically about Futura133, but I've tried many boards with sails larger than recommended (one of them Tabou Rocket 135, somewhat similar to F133). I'm heavier than you (85kg) and my experience is - it didn't bring me any early planing advantage, it just bogged the board down and it was harder to handle. You are lighter so you could gain something, but I still think that it won't be worth the effort (and cost). And quoted sail ranges are already a bit optimistic in my opinion (I wouldn't want to put anything bigger than about 9 on a F133/width 76/tail 49). To really gain a SIGNIFICANT advantage with a 10m (over your 8.5), you also need a bigger board/more powerfull tail/fin, etc.
PS With your weight, I'm guessing you are able to plane with your combo in steady (not holey) 10/11 knots. Below that, it's really a Formula or a longboard territory.
Last edited by Screamer; 8th September 2009 at 10:16 PM.