As you yourself point out, getting the 70-200mm f/2.8L IS, will give you a wider aperture lens, always useful in terms of both low light and depth of field. I don't think you'll find the IQ of this lens significantly better than that of any of the lenses you currently own, at least in my own experience (I personally think the IQ of my 70-200mm f/4L non-IS is better at the edges than my f/2.8). If you really want absolutely outstanding IQ particularly in terms of sharpness, you should consider the telephoto primes. But that starts to get very expensive.


Remember that version II of this lens is about to hit the streets. That may well be a knockout IQ-wise. Probably worth waiting to get the word on this before you buy.