RE: Pros and cons of smaller sails on Formula
Well, even a drafty freerace sail is "small" at 8.5 when using it on a fw board. There is no doubt that you would gain both early planing (even though what you get with the 159/8.5 beats your S-Type by a lot) and angle. I especially find that what I get from larger sails is angle.
Beware that race sails are build to cover a "full" windrange from say 10m2 to 12m2. That means that a 10m2 race sail might only have similar grunt to your 8.5 whereas an 11m2 might have a whole lot more. For instance I am pretty much left to watch others planing with my 9.5 Vapor if the wind drops much below 10m/s (I'm not a great pumper and 95kg) but on the other hand I'm pretty comfortable on that sail in say 13-14m/s. In comparison I once tested my North MatchRace 8.8 on my old 159 and easily got going in winds that I believe to be in the 11-12m2 formula range. Then again that sail started feeling weird and backwinding etc. once the wind picked up. It did actually trim quite ok which surprised me. While not as comfortable as with a 10+ I did not really feel that uncomfortable with this size of sail on the board so I believe you can easily find a comfortable stance with this size rig.
But the real difference is what happens once you get your gear going and apparant wind increases. From that point you can really benefit from the stable foil of say an 11m2 race sail pointing a lot higher in low winds than with smaller sails. Whether that means more fun or not is a different question. To me it is; searching or optimal speed/angle is a great part of what I find fun about WS.
It is really hard to say whether YOU will have more fun by getting a larger sail. If TOW equals fun, then YES - but then you either need a 9.,8-10 drafty freerace or an 11m2 race sail (as noted earlier a 10m2 true race sail won't do). At your weight I would suggest the latter as that would mean a massive range in one sail. An 11 will get you going in all but the lightest stuff and as long as you're not trying to follow any fw racers upwind in light stuff it'll be plenty large. Many sailors believe that 11's pump better due to slightly better handling. Add to that the decent seconjdhand availibility of such sails compared to large freerace rigs (that people tend to hang on to). For instance DEN-13 is selling a Vapor11 right now I think, a sail that I'm pretty happy with also for formula freesailing.
The big question here I think is whether you're happy with the TOW you can get from your 8.5 right now. Even though you're planing way earlier than with your S-Type I kinda doubt that you get going in much below 7m/s or so (I could be wrong though) and there are plenty days in Denmark with less than that during summer. If getting on a plane and blasting about in 5-7 m/s will translate into more summer fun I suggest trying an 11m2 proper race sail. Possibly you can borrow/test one locally once or twice to see whether you feel it's worth it. If you decide on such a sail I sort of believe you will use it a lot more than planned as it'll be pretty comfortable to a least 10m/s with a rider of your size/skill.