The 28-300L is convenient for travel. If you mean the MkII versions of the 24-70 and the 70-200, the IQ will be much better. Yes, you can shoot them wide open. I suspect you'd be fine cropping the 70-200 II images vs. 300mm, or you could add a 1.4xIII down the line for a 280mm f/4 lens that's still a stop faster than the 28-300 at 300mm.