Nikon 70-200 and 2X teleconverter
I've heard reports that the new 70-200 and new Nikon 2x teleconverter works alright. The earlier TC20E II and 70-200 had some softness that really defeats the purpose of great glass. The 70-200 works great with a TC14E II and pretty well with a TC17E II.
I think its a false economy to put a 2x teleconverter on a 70-200 lens. There are a lot of combinations that provide more reach without the image degradation. The 300 f/4 works great with a 1.4 teleconverter and even the 1.7 in good light. On a tight budget the Tamron 200-500 is a reasonable option - as is the Nikon 80-400 or Sigma 50-500. My wife uses the Tamron 200-500 over the Nikon 200-400 because it is so much lighter and still produces good images. A friend has bought and sold the Nikon 200-400 twice in favor of the Tamron due to the weight.