My "walk & stalk" package is the Canon 100-400L II telephoto zoom + 1DxII. I very seldom use a teleconverter with it as it slows the auto focus too much for my liking, especially when photographing birds in flight.It can be a challenge to go out and do any outdoor photography. I was and still am planning on picking up the Canon 100-400 II as a better low light lens. To get the reach of the Tamron, 600MM. I would have to use a TC and will it be any better than the Tamron lens?