Interesting thread, and thanks for posting the sample photos.
I don't have either 300mm f/2.8L IS or 400mm f/4L DO IS. However, as a prospective buyer of a 400mm+ lens, I am drawing following conclusions from what I've read/seen so far:
1. With 300mm f/2.8L IS + x1.4TC (420mm f/4), you can achieve as good (if not better?) image quality as the 400mm DO. What's not so clear so far, is how 300mm f/2.8L IS + x2TC (600mm f/5.6) would compare to 400mm DO + x1.4TC (560 f/5.6).
2. 400mm DO is a great option if you're really looking to use it at longer focal length, x1.4 (560mm f/5.6) or x2 (800mm f/8).
3. Cost difference between 300mm f/2.8L IS and 400mm f/4L DO IS is still pretty significant. $4100 (300 2.8) vs. $5500 (400 DO) => $1400! Even you throw in the TCs, the difference is still more than $1k. You could buy 300 2.8 and 400 5.6 for the price of 400 DO (although that's pretty unlikely). These lenses weigh about the same, but 400mm DO is easier to handle, as it's shorter.
So, in all, unless your purposes are 1) to be able to hide your 400mm under your coat (i.e. short length), or 2) obtain a poor-man's 800mm, albeit at f/8, I can't see any additional benefits of 400mm DO over 300mm 2.8, given the cost difference. If Canon lowered 400mm DO's price by $1200 (i.e. to $4300), then I think it could become a really logical alternative to 300 2.8 + 1.4TC, if you indeed shoot always at 400mm+. However, I can imagine Canon's investment in getting DO lenses to work is rather high, considering limited number of models they have, and as their success isn't probably as wide-spread, unless Canon is going all DO in the future, I doubt they can justify lowering its price.
What do you think?