Teleconverters

How much image quality is lost because of a good teleconverter? Is there any data to quantify the loss in percentage?

There is no data that I know about teleconverters. However, all teleconverters degrade the image a bit because you are adding additional number of glass or plastic elements behind a lens and the light is likely to bounce/reflect/refract though the additional elements and not exactly focus/converge on the focal point. However companies design teleconverters keeping their own lenses so that the degradation is limited. So the Canon TC 1.4x III and 2xIII are designed to better work with the newer telephoto lenses.

I do have both the Canon EF 2xII and the EF 2xIII TCs. I haven't tested them side by side. However, based on the images, my perception is that the 2xIII is slightly better. Earlier I used to use third party TCs but then gave them up after I saw the difference between dedicated Canon and third party TCs. Hope this helps.

Thanks Sabyasachi for the detailed explanation. Somehow nobody publishes MTF charts for any lens, before and after teleconverter.