I have been shooting with a 400mm DO for several years. It is a great best bird flight lens for hand held shooting. For me, my best 400mm DO images seem to be taken against open sky with no distracting backgounds. The lens has a rather strange boken pattern which can be very distracting. It also does not treat bright specular highlights very well. I have had to use Gausssian Blur to clean-up some backgrounds - lots of extra Photoshop time!!
Despite the shortcomings , I continue to shoot the 400mm DO because of the combination of relatively light weight and BALANCE. The lens is not front heavy as is the 300mm f/2.8 which is probably the main reason that it is so easy to hand hold. In comparison, the 300mm f/2.8 with 1.4x is rather front heavy and not much fun to hand hold.
I would not recommend the 400mm DO to you without knowing your application. It is a specialized lens with definite limitations. Based upon my tests, the lens is about as sharp as the 300mm f/2.8 with a 1.4 extender. The 300mm is sharper when used without the extender. The 400mm DO works just O:K with a 1.4X - not acceptable to me with a 2X.
If you can live without IS and f/4 - the little 400mm f/5.6 lens would be a good candidate. Try it - you can always find a buyer if you don't like it!