The short answer (see that other thread for more of a discussion) is that with the TC14E image quality is minimally harmed — you almost cannot tell the difference TC off or TC on, except that you are that much "closer".
Alan, you typed "TC20E II" — that's "two" — and if you mean that, forget it. You want the current version "III". I find the results quite good, especially if you can stop down. The tradeoff for some loss of IQ, of course, is you have a handholdable 400mm!
Jon Kandel A New York City Nikonian and Team Member Please visit my website and critique the images!