Cropping and using pixel enlargement to go from 200-400mm equivalent you retain the DOF of 200mm. Using the 2X TC you have DOF of 400mm ....that is my experience - I am not quoting theories or internet "sources" - so don't ask me to provide black and white "data" for this.....
Although this discussion has gone quiet, the item below from Artie Morris came in via email this past week. The next time there's a discussion about Nikon 2x teleconverter and a 25% loss of quality, expect this link to appear as a response within the discussion ...