In retrospect, it seems odd that the 400mm focal length has been the test bed, as working on the longer lenses would be a better value proposition. Price cannibalization doesn't seem like the concern, as - it being Canon - they could just charge $18k for a 800 DO and make even more of a margin with it.
The number of people looking for a 400 vs 800 is huge. 400 users have tons of options each with advantages but huge disadvantages too. We've long been asking for a mash-up of these to get an affordable, lighter, faster, IS 400mm. The 400 DO II seems to be the Canon reply.
Riker, kind of all over the place in your comparison. The 300 II did get .5m (~1.5 ft) better mfd but the 400 DO II got better too ~ 1 foot better. The 300 was already about 1 meter better so it has a 4.2 ft advantage. For some this is a real advantage for others not so much.
If you need the 300mm that is an advantage but if you want 800mm sorry, the 400mm wins on that end. Personally, if other options were available, I would not buy a lens with the intention to always use it with the 1.4x or 2x attached. I prefer to use a lens at it's native focal length.
You compare the 400 to the 300+1.4x but when comparing weight you didn't add the 225g of the 1.4x.
The 400 DO II sounds like a really nice combination of IQ on par with the other supertele-primes, IS, light weight, faster than the 5.6, and lower price. Certainly not cheap, I hope the price will drop some, but it is better than the 10,000-12,000 of the STPs.
Again, to me this sounds like a great reply to what people like me have been asking for. If you already have the 300mm+1.4x you have a great rig and, if you're happy, no reason to think about the 400 DO II. I'll take the 400 DO II because I feel focal length challenged and the odds of my wife saying ok to a $7000 lens are much greater than a 10,000-12,000 lens.