I can see that you already set your mind to the 137, but especially with your weight I find the gap between the formula and the 137 too small.
Like Remi already said, if you don't use the formula for racing (very sharp upwind racks with the highest possible speed), you should really consider the US147 because it would replace the formula + the 137.
It would carry all your sails 7.6, 8.6,10.0 and even 12.0 perfectly. That's what this board is made for.
Personally I use the US147 with 8.8 and 10.9 @87kg. I can tell you that it carries the 10.9 very easily, so it will also carry a 12 without any problems. Furthermore it feels incredible light under your feet c/w formula and also handles smaller sails like a charm. If you could test it somewhere, I'd really advice you to do so. It's probably one of the best light wind slalom boards on the market.
About wood / carbon, I would definitely go for carbon. After all, you're choosing a board for light wind when there is almost no chop to absorb.
IS87 / IS101 / IS122 / US147