I'm not familiar with the S&S carb, so I can't say for sure. The problem exists with the stock CV unit. When you remove the air cleaner and look into the carb, on the left side is a little vent for the float bowl. On the newer Forcewinders there is a little notch cut out so the vent can get air. (I'm taking pictures now. I'll post later today.) Unfortunately it's no where near big enough. Older Forcewinders had a large section of the inside of the pipe machined away, allowing the vent to breathe properly. What I did was take my Dremmel and hone out the Forcewinder in the area of the float vent. This helped, but it still runs a little lean around 5,500rpm and up. I'd check with Forcewinder to see if their product will fit the S&S carb and if there are any issues with the float vent.
The stock Sportster carb has a ton of power potential for very little $$. Any other carb I've tried kills low end torque in exchange for top end HP. I run the same stock carb on my 2000 Buell (1203 Sportster engine) and have had better results than bigger more expensive carbs. If you plan to go with the S&S, consider a hotter cam to take advantage of the bigger carb.