I had the combo but have now sold the 2xEXIII. IQ was fine as well as AF. However, I found the combo to be heavy and cumbersome. Now I am using the 70-300L in those situation. Quite a nice lens too. Thinking of getting the 1.4EX as it would give a f4 lens and even could be used with the 70-300 at the long end although at f8. Tried the 2x with the 300 f2.8 II for a couple of weeks. That is where it excels. Unfortunately, I don't have the 300 f2.8 lens in my bag.
Had the 2xTC III for use with the 70-200 II, but felt I just ended up with a cumbersome heavy combo. Probably should have chosen the 1.4 EX instead also giving me f4 and not 5.6. Had to send the lens to Canon as the lens new out of the box had a defect rubber gasket at the lens mount (part missing). Also I am not that impressed on the sharpness. It is great but not great as in fantastic.
The 16-35 IS is a very fine lens indeed. Sharp, crisp, quick focus, superb IS. Light too if that matters. if you need a UWA Canon, clearly this is the way to go. However, UWA requires skill and talent. Personally, I am still in the very begin of what I hope to be the learning curve.