So if I catch the logic here is that on the big white lenses the teleconvertor degrades the image by a certain amount, say double from 2% to 4%. Since the 100-400mm starts much worse say 4% degradation it ends up doubling to 8%. Those numbers do not relate to anything and are just an example.

I used teleconvertors on the 100-400mm and 5D II when I had the lens. I never got a product that I would do a large (over 8x10) print with.
It sounds to me like you are going to be thinking about one of those big white lenses shortly.