From what I was informed on from our purchasing person is that the colors from the IPF8300 are the exact same colors in the IPF8400. Same cartridge but different chip so you couldn't use an ink from the 8300 in the 8400 and vise versa. I'm just stating from what I heard. Our company had a training session with our Canon rep, but I was out of the office that day. I will know more about it next week when I get my training on the new series. I wish I could be of more assistance now, but will know more next week.
From what I've heard from Scott Martin & others, the inks now come in a new cartridge that is backwards compatible with x300 machines. I suspect the reason for the new cartridge is to accommodate using 300ml cartridges in the 6400.
So you can't use the old cartridges in the new printers, but you can use the new cartridges in the old printers. It only makes sense; putting the same ink in two cartridges would be a silly waste of money as far as manufacturing costs.
As for the dithering and other improvements, it's really hard to tell from the marketing materials how much of the improvements only apply to the 9400 relative to the 9100. Even if there have been changes to the dithering across the entire model line, I think it will be a small change considering that the heads and pretty much all the other hardware remain unchanged from the x300's.