Concerning tdpriest's question "...where, then, is the advantage?" in the comparison of using the 200mm Micro-nikkor with the 6T close-up lens for super-macro subjects vs the 105 VR with the Nikon's new AF-S TC20EIII.
As I have said, I have not shot what I consider a definitive set of comparison shots to determine if there is any difference in sharpness. But my "gut" feeling is that the 200mm MN with the the Nikon-made dual element 6T close-up lens would be sharper than the 105VR + 2.0x T/C.
Beyond the issue of sharpness, there is the fact that the viewfinder brightness of 200mm + 6T is significantly better than with the 105mm + 2X. According to the effective aperture readout shown in the D800's viewfinder, the wide-open effective aperture of the 200mm +6T at 1.8x life-size is f5.3. With the 105VR + 2X the wide-open effective aperture at 1.8x life-size is f9. And f9.5 at 2x life-size. This is a brightness difference in the camera viewfinder of 1.67 f-stops. I think this brighter viewfinder, which also exhibits less depth-of-field, will make it easier to find and frame your subject. And focus more accurately if you are using manual focus.
Additionally there is the issue of auto-focus accuracy. Nikon states fairly clearly, that their auto-focus system works best when the maximum aperture of the lens, or lens + accessory, is f5.6 or wider. The 200mm MN + 6T meets this criterion throughout its whole focusing range, from infinity (f4) to minimum focus (f5.3). The 105VR + AF-S TC20EIII does not meet this criterion anywhere within its focusing range. My D800 says the wide-open effective aperture at infinity is f6. This is probably why Nikon originally stated that the 105VR + 2X did not work. As we know, it will auto-focus, or at least try to auto-focus. But as you focus closer and closer, the wide-open aperture 105VR + 2X the drops downward towards f9.5, at minimum focus. And I would believe the auto-focus becomes less and less accurate, particularly when the ambient light levels at lower as they frequently are underwater.
Those are two answers to tdpriest's "where, then, is the advantage?"