My experience is that my 50/1.4 is wicked sharp at f/4 and above, and down to f/2.8 its still very nice. It gets really soft when its open more than f/2.8, which seems to match everyone's experience. I have a had time picturing how you'd improve significantly on its performance above f/2.8, though... I imagine the ring-type USM might make it faster focusing, though, and FTM would go along with it, so... that might be cool, but maybe not worth the upgrade price, for me, if that's all I'm getting...