To your questions: the lens has the best resolution around f/8. This was a test to see how the lens did with an obliquely backlit subject , wide open (f/4), and how well the VR works when the lens is handheld. I can and will be getting my hands on a 70-200mm f/2.8G VRII but have not planned on the teleconverters (interesting idea) and definitely not the 80-400 Nikkor. Sorry.
I'm surprised you find the lens sharpest at F8. This does not augur well for its usefulness with a teleconverter.
My delay in deciding whether or not to buy this lens is not just related to its overall sharpness, and particularly its sharpness at F4, but the usefulness of its FL range compared with my current lens, the Nikkor 24-120/F4. In other words, those increased focal lengths from 120 to 200 may be less significant and less useful than the loss of focal lengths between 24mm and 70mm.
If I'm going to sacrifice those very useful focal lengths between 24 & 70mm, I feel I need a longer maximum reach than 200mm, which is why I've been interested in the new Tamron 70-300/F4-6.5 USD VC.
A thanks to Luke Kaven for mentioning that Photozone have just recently tested the Nikkor 70-200/F4 VR on the D3X. I can now compare the lens with the Tamron which has also been tested by Photozone on the same camera.
The Tamron lens is considerably cheaper, marginally lighter, and has a reach of 300mm at F5.6. One certainly wouldn't expect it to be nearly as sharp as the Nikkor at any focal length that both lenses have in common, so I was very surprised to see that the Photozone results show the Tamron lens is in some respects sharper than the Nikkor at 70mm. Refer attached comparison charts.
Whilst the Nikkor is marginally sharper at F4 (3744 as opposed to 3635 in the centre), the Tamron is sharper at both F8 and F11, and very marginally sharper at F5.6. However, I'm sure such differences are within the range of 'quality control variation' amongst different copies of the same model of lens, so one shouldn't place too much significance on such minor differences.
At other focal lengths, the Nikkor begins to pull ahead and at 135mm is clearly sharper in the centre a F4, than the Tamron is at F4.5. But again, the Tamron is marginally sharper at F8 and F11, in the centres and in the borders.
At 200mm one sees the biggest difference. The Nikkor is sharper at all apertures, including the borders, with the exception of F11 where the Tamron is very marginally sharper to a degree which is of no consequence.
What I see at 200mm is that the Nikkor should be visibly sharper only in the centre at F4 and F5.6. Differences at the borders and extreme corners are very similar for both lenses at all F/stops.
At 300mm, the Tamron takes a dive, but at 300mm it's still sharper than the Nikkor 80-400 is at 400mm, particularly at the borders.
I'm going with the Tamron plus a 24mp D3200. Both lens and camera together will cost about $800 as opposed to $1300 for the Nikkor lens alone. The greater pixel density of the 24mp cropped format sensor should ensure I get more detail from the Tamron lens when maximum reach is required, than I would get from either the D800E with Nikkor lens at 200mm, or the D3200 with Nikkor lens at 200mm.
In other words, the 50% increase in focal length (from 200 to 300mm) should be worth more than the 10% increase in resolution that the Nikkor lens has at 200mm and F5.6 compared with the Tamron at 300mm and F8.