It is a dramatic improvement, and they seem to have solved the contrast problem. However, it is now only 250g (1/2 lb) lighter than the 300mm f/2.8 II and so you pay your money and takes your choice between two outstanding lenses. I'd love to try the 400 DO. 800mm is very attractive with a 2xTC but we'll have to wait to find out what the AF is like at f/8.
For some (me included) this might be a Big dilemma between excellent 300mm f/2.8 II and 400 DO II ( looking promising). Can't wait to see first reviews and pictures from production models