With regards to the option of using one of the 300's with TCs to get the reach. Having both of them I can say the f2.8 Vii plays much better with the 2x TC than the f4 does with the 1.4x TC. I am finding that when stopped down to f8 or more, that there is only a small amount of CA and distortion which limits the extreme cropping options.

It may be out of your preferred budget range, but has anyone heard anything about the 200-400 f4 with 1.4x TC built in?