For something more complex/expensive like a 70-200vr, I would want it to be a US lens so that Nikon could work on it if anything goes wrong.
For simpler things like a TC, I see no reason why Keh or some other reputable non-Nikon repair shop couldn't handle a problem.
I bought my TC-14EII last year from a Hong Kong seller. It works great with my 70-200vr, but I sometimes wish it could get me longer. At that time, the TC-17 was newly-released and I didn't want to take any chances with image quality. Everything I've read since then suggests that the TC-17 nicely balances image quality and focal length extension.
I perceive the price disparity nothing more than insurance.
Many lenses that are "gray" are USA lenses in USA boxes...most recently my B&H 3004f gray.
I'd not buy a body non-US because of prohobitive repair costs.
My Nikon authorized repair has no hesitance to repair gray marked stuff privided, of course, I pay their usual charges.
1.7 TC, 105 micro, 30f4 gray.
D2X, 70-200 etc, USA.
I just bought the gray 1.7 from Adorama. The quality is fine. It is an acceptable risk in this situation. I recently shot for a day at Niagara Falls using the 70-200VR with the 1.7 TC. I'll post some shots with the TC shortly.
I checked with Melville on a couple of lenses. They have I believe it was 3 levels of repairs, with an ascending price. I found most times as in buying used, if you ran into a big problem, you could have bought new.
But in new, I think melville just puts the little USA stickers on the product and the box, otherwise it is all the same. CS Dayan