Having had the new 300mm for a while now I must confess that I use it a fair bit with the 1.4x TC at sports events when I am further away, am wanting to reduce distracting backgrounds by reducing the FOV, or am trying to get some tighter portrait style shots which is an interesting niche to be able to provide. It works well enough with the TC on it that it is becoming 2 lenses in one for me.

Would I want to trade it in on a 400mm? Definitely not!! The native 300mm focal length is too often used. Would be nice to have both but then I'd want a pack mule to help carry everything around too!

The only other interesting thought that has occurred to me is to get another 1.4x TC to allow having one on each of the 300mm and the 70-200 when occasion calls for it. The 200-400 would be nice, but not available to purchase.