I also heard the 80-400 is poor from 300-400 which is the range you would want if you get the lens...otherwise there is no reason getting it with such a large focal range. I personally have thought about the 300 f4, but the teleconvertors won't allow auto focus and it might loose too many f-stops to be useful anyway. There really isn't a "good" option under $2000. The closest would be the 70-200 with a teleconvertor, but then you are in the $2500 or more range.
This is the only reason I am even considering any other brand lens then a nikon lens...and I still don't know if there are any good lenses by sigma or tamron in this range.
So it seems the 80-400 is out, possibly soft at 400, and can't use TC, so can't get past 400mm. Plus for me, where I already have the 80-300, it is not worth carrying another lens for just the 100mm.
The 300mm/f4 is too short to begin with, you would always need at TC. So that seems like a 'no go'.
70-200 is good, but even with 2x TC can't get past 400mm, so that's not an option.
Gelu88 suggested I look at Sigma, and others here also suggested that. Looks like Sigma has zooms that might fit the bill (oh, really bad pun, not intended). At 400 or 500mm I would be just as happy with a prime lens, I think. I would use a TC as time and need allowed. But if the IQ is OK, then zoom is OK if the price and TC use are reasonable.
From all the posts, it appears there is a market. I hate to see Nikon give up this market, but I also can see that it might be hard for them to compete with Sigma, as Sigma will take the same basic lens and market it for all bodies, thus a larger market. Maybe there is a Sigma lens in my future. Certainly not what I expected, but I am very new to the DSLR game.
My prior post about the 80-300 not being good enough was only in reference to birding at 100 or 200 yards. I have that lens, love it, and it almost never comes off my camera.