A little experiment tonight - I put the 2.0x Tamron teleconvertor on the back of the 600mm lens for 1200mm of focal length on the DX-format, Nikon D500. Focusing is a bit more challenging, but the result was notably better than the 600mm version. I've tried this experiment a number of times on different camera bodies - this is by far the best result I've had with the 2.0x on.