CarlI have been searching for the images I took to illustrate my post but I have to admit I can't find them - sorry! They may be on my old PC (which I still have) so I will see if they are on there.
What do you think of the 400 DO outresolving a 600 f/4 L?
Basically I fancied one (but couldn't afford it!) so I tried out the 2 at my local camera shop. As a comparison I then mounted my Canon 600 F4L IS and took pictures of the same subject (a BMW 3 Series) from the same place. The difference was that we could see that the lacquer on the rear of the car had crazed slightly with the DO lenses we weren't easily able to pick this up with the 600.
As I said I will have another look for the images.
The 400 DO is certainly a very controversial lens - but the ones I have tried have impressed me though less so with extenders.