jwhphotos wrote:
You don't mention which camera you are using. The camera body can make a huge difference in focus speed.
Myself, I use the Tamron 150-600 I on either a Canon (I know you are using Nikon but) 7D or 70D and the focus is more than fast enough for flying birds. The sharpness is also quite fine and any improvement brought about by any other lens, even a prime let alone the new Tamron or Nikon 200-500, would be small and not worth the purchase price.
Just some food for thought.
Oh sorry I meant to but must have forgotten. I have 2 bodies, Nikon D750 (full frame) and D7200 (crop frame).