Interesting because I have the 70-200/4 IS now and also the 135L. I have been thinking about upgrading the f4 to the new 2.8 II but I know I am going to buy a FF after the new 6d2 comes out: if not that then either the 5d3 or 5d4. Thing is I would get more than a one stop advantage going FF from crop in ISO from my 7D than I would get going from f4 to f2.8. Plus I would get to keep the lighter f4 on the camera that way. But I still want the 2.8.
I already have a 5D3 and needed more action-stopping light which prompted the 2.8 II purchase.