The carbon FSA cranks specify the BB8681 which I understand is slightly narrower than HTII and the MegaExo BB specified for the non-carbon FSA cranks. The effect of not using this specific BB is potentially too much bearing load with crank bolt torqued to spec. You might also have difficulty getting the crank spindle to fit the HTII BB as although both nominally 24mm, the FSA spindle is marginally bigger than Shimano's. I'd be interested to know how you get on with the Ultegra BB as they're cheaper and easier to get hold of than BB8681s.