The quality of the images using TC it is related to the quality of the lens used.
The 300mm f/4 with the TC x1.4 it is an excellent combo
The Nikons 500 and 600 f/4 work very well with the TC x2
The performance of the Nikon 200-400 with the x1.4 and x2 is very good as well.
There are heaps of excellent wild life and avian images in the pro forums to back up this.