In order to balance the fighting style with TWF, THF and arguably S&B, the bonus that SWF gets you when you have all 3 feats should probably be 1.5 you damage stat modifier.
-edit- This is more of an illustration of how a topic that has had a very heated discussion before was silenced not by changing the mechanic but by changing just the description of the mechanic. People went silent about it and probably many switched from SWF to THF, while in actuality SWF stayed as good as it was before.
In other words, descriptions matter.
While I think balance is important and we should have interesting choices, I actually want this ability to be changed just to match it's description, because it is even more important to know what you are choosing when offered a choice.