I tested the Schneider 2X Teleconverter with 150 mm Schneider Kreuznach lens with my IQ160. I found out that it is a very good combination with very good sharpness.
The best is that it works with the Mamiya 4,5/300 lens, too. And the quality is not too bad. But I'm not sure if a Nikon D800 with a 400 mm lens isn't the better choise.