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?"
Fred