I have experience with both the 1.4x and 2x TCs on both the 200-600 and 70-200II. On the 200-600, forget about it. The loss in resolution is too drastic in my personal opinion, just stick with the 1.4. Now, the 70-200II actually handles it nicely in certain situations. Obviously, you'll see some drop in quality but I've gotten good results with it. The 2x really only works well on the 70-200II, 400GM, and 600GM.