Rodolfo Paiz
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Humanoid wrote:
A 300 VR lens or 300mm f4 AFS with TC17eII is 510mm and very sharp while maintaining AF. Though, the f4 lens will focus much slooooower the higher ISO capabilities of today's DSLR's allow us to get the shutter speeds to an exceptable setting.
The part about "slooooower" AF is simply not true, and is a statement typically made by those who haven't used the 200-400. The 300+1.7x yields a 510/4.8 while the 200-400+1.4x yields a 560/5.6. The zoom gives you 50mm extra, but loses half a stop. The AF on a D300 does get a little slower and less accurate at f/5.6... but the same thing happens at f/4.8 and there's very little difference between the two. I've shot both combos enough to know (especially the 200-400, through which I've put at least 60,000 clicks).
The AF speed/accuracy difference between the 510/4.8 and the 560/5.6 is just about the same as the difference between the naked 300/2.8 and 200-400/4: perceptible and real, but small enough to be negligible. It only matters when even the tiniest bit of speed matters... and if it matters that much, then the shooter should either be making enough money from his work, or care enough about his hobby, to really spend the money on the absolute best gear that could possibly be had at any price: and that would include pretty much all the big guns, since each serves a special purpose if that's what you need.
Also, please note that I compared the performance of the two lenses on a D300. On a D3 or D3x, the difference in performance is smaller because the camera's servos are stronger and the AF slightly faster and more accurate. Some of my best shots ever are from a trip to Africa with the 200-400+1.4x, even on a D200. 
Humanoid wrote:
Well...still didn't answer my question as BIF is critical and handholding a 200-400 is not a problem but handholding a 600 would be and forget hiking with it . So....a TC on a 200-400 would give the range needed but would it sacrafice IQ or AF tracking?
Short answer: no. The AF performance and image quality of the 200-400, naked or with the 1.4x TC, is so close to matching what you can get from the 300 and 400 primes, that there is no functional difference. The only differences are the real-and-perceptible-but-small-enough-to-be-negligible performance delta I mentioned before and the maximum aperture you can get from each. If you absolutely need f/2 at 200, f/2.8 at 300 or 400, or f/4 at 500, then you need the prime. But I don't think you'll find yourself in a situation... any situation... ever... where the performance delta in AF and IQ between this zoom and any of the 300+ primes will be enough for you to say that you absolutely needed the prime to get the shot.
In fact, you've just made a clear case for the 200-400 yourself. Even if the 600 produced perceptibly sharper and perceptibly better images (which I'll argue it really doesn't), then that's still entirely useless to you if you can't get the shot at all because you can't lift it, right? So yes, sharpness is a factor, and AF is another, and IQ is another. But in all of those factors, the 200-400 holds its own extremely well. In the area of reach and maximum aperture it is beat by the primes, but in the area of handholdability and portability it excels.
Pixel peep all you like... there's plenty of images out there to look at. What I'm telling you, in case that opinion is useful to you, is that pixel peeping really is irrelevant and you'll end up wasting your time (at best) or making unwise decisions (at worst). Figure out what you need from reach and aperture, from weight and portability/handholdability, from convenience and cost, and those will tell you directly which lens you should get.
With any of these lenses, the limiting factor in image quality will be your own skill, not the lens. That's certainly true for me and for most shooters I've ever met.
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Glad I could add to your torment 
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