Supposedly, the D600 can AF down to f/8 and -1EV. Pretty amazing, though pushing the limits of technology. Might be better to manually focus or use an IR AF focusing aid (such as the ones on accessory speedlights).
I don't think a TC on an 18-200 makes much sense, I think a better solution would be to get a faster or longer reaching tele zoom lens.
TCs are tools, and should be used under specific conditions, they can degrade image quality vs. a lens used without a TC, but do add some additional reach at the cost of speed. They shouldn't be used all the time, however.