I have the 1.4x mk.3, but I have never used a 2x TC.
I find the 1.4 x does not do a whole lot on my FF camera with a 70-200 lens, as others have said here. But it actually did make a big difference on my crop body, as long as you are not
shooting tiny things like birds or squirrels.
My feeling on birds is that they are terribly cheap, and will seldom pay much for even a great portrait of themselves. Spending $7k on a bird lens would take forever to recoup in shiny bits of junk and sunflower seeds.
My kindergartener plays soccer on a midget field, and the 1.4x would make the lens too long to shoot that.
Come to think of it, she never pays anything for her photos either!