Ok as all is quiet I decided to try and give the Nikon MKlll 2x Coverter a test. I had it coupled to a D4 body and 500mm lens. All images were shot at ISO 800 in Aperture Priority adding exposure compensation in as required. Overall it looks quite good, not as sharp as the 1.4x converter but acceptable. Image results below:
I missed the focus on the Bittern in the third image and the Peregrine looks a bit soft, but it was hand held.