A couple of weeks ago, I picked up the Nikon TC-20EIII 2x teleconverter. It allows me when the teleconverter is connected to my 70-200mm f/2.8 VRII, to shoot at 140-400mm f/5.6. The only problematic part is that the autofocus speed goes right down when the 2x teleconverter is attached. You will not be able to track moving birds with this particular lens unless you prefocus at a certain focal length and hope to hell you track it enough to attain focus on it.
The TC-20EIII has an aspherical lens element that allows the TC-20EIII teleconverter to get a sharper image than the previous iteration (TC-20EII) but still has the problematic "softness" in most images. You will have to stop down 1 full stop to f/8 in order to regain sharpness. That pretty much puts this lens in the realm of manual-focus "prefocussing" in order to get the image that you want if it's action. Ideally when it comes down to it, you want to have either the 300mm f/2.8 VRII or the 500mm f/4 for birds in flight as the lenses though difficult, are a darned sight easier to handhold than the 400mm f/2.8 or the 600mm f/4.
When it is attached to the lens, in terms of overall lens length, it adds a full 3 inches to the 70-200mm f/2.8 VRII lens, however the weight difference is negligible. It feels roughly about the same weight as it usually does barebones (without the teleconverter).
Here are some of the shots I've managed to get from this lens teleconverter combination. While I am primarily a wildlife photographer, I do shoot hockey (sports) photography for my son and others.
Overall, the lens-teleconverter combination is slow to focus, so you have to prefocus manually, so the best way to utilize this lens is to put it in the M/A mode handheld or with a monopod, if you are shooting handheld. Keep the lens on active focus and your camera on zone focus.
Understand that the TC-20EIII is a teleconverter that isn't meant to be the absolute solution for extending your range. The TC-20EIII is meant to be an economical way to get to 400mm on the 70-200mm f/2.8 VRII and is a great solution for the hobbyist. If you are shooting professionally. Spend the money, go for the 400mm f/2.8 or the 500mm f/4. There is no substitute for prime supertelephoto lenses and you are short-changing yourself if you're shooting professionally.