The IPF8400 will be practically the same quality as the IPF8300 since the inks are exactly the same. It was updated in the hardware and software. The Epson 9900 is supposed to be the top of the line series due to the HDR colors, but is pretty close to the IPF8300. The IPF8400 is now available and the 8300 is discontinued from the end of last month due to the SGIA show.
There are small quality changes in x400 - it has reduced graininess of skin tones in Highest Quality Print Mode, and it's supposed to produce smoother gradation in B&W.
New iPF8400 is 10% faster in Standard and High quality print modes, and 100% faster in Highest quality print mode, while maintainig the same (or even a bit better) quality. It's also more convenient that all Print Quality Modes share same ICC profile, so it's faster to choose the right ICC profile and there's less chance for potential errors.
Over advantages of new series is improved accounting function and Canvas Wrap, Variation Print features in Photoshop Plug-in, higher capacity HDD, and newer built-in spectrodensitometer.
Ironically, Canon LUCIA EX inkset has higher dynamic range on photo papers than K3 HDR. Higher black dmax provides more sense of depth, so prints are more "3D-looking". The color gamut of both inksets is about the same.