What camera are you shooting with? With most you are not going to be able to auto focus. With a 1.4X extender your max aperture would be f/5.6, but at 2X you loss two stops wich puts your max aperture at f/8, and most cameras will not auto fucus. If you have a 1D series you would be OK, but you are going to sacrifice a lot of shutter speed. Considering you want to use it for sports and wildlife, I do not think you want to sacrifice that much light (shutter speed). If you were looking at the 70-200 f/2.8 IS II, then maybe it would be worth it, but the cost goes up a lot. Basically, the 100-400 may be better choice, and with wild life, the 200 mm is kind of short. Are you planning on shooting over 200mm a lot?