A teleconverter isn't going to make your shots sharper, that's for sure. Generally you need to stop down from f5.6 to get the best results... and that's on top of the f-stop of light you've lost already. So it really is a fine weather accessory. Also remember the stability factor, you'll need some mighty fast shutter speeds if you're going to handhold it.
A good tc like a Kenko pro won't degrade the image in itself too much, but its the side effects that cause the problems.
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