I've used the Kenko 1.4x teleconverter on the 18-200 with no problems. It seemed to autofocus OK, and image quality seemed good. I haven't put the combo to much use because I have longer lenses. I just tried it to see how it worked and it seemed useable.
On lenses I actually have used the Kenko on more (55-300 and 80-400) I have found that the lenses will autofocus without problems in good sunlight, but even in overcast daylight conditions autofocus becomes iffy. So, I'd expect the same with the 18-200.
Overall my view is that on slower lenses a teleconverter is an inexpensive way to get more reach if you need it, but not as good as having a longer lens. The 55-300 is just a bit more expensive than the Kenko teleconverter, but a much better way to get that extra reach.