Dxomark tested the A6400 sensor and there is really a small difference:

Anyone comparing the Sony A6300 and A6400 will find the results intriguing. Although the two models share a similar sensor resolution (pixel count) and the overall results are very close, the measurements are not identical.

Despite its slightly higher noise floor over the A6300, the new A6400 sensor performs well, combining a wide dynamic range over a highly useful range of ISO sensitivity settings and excellent low-light ISO capabilities. Although that directly impacts the color depth, the A6400 remains capable of delivering extremely pleasing images. Combine those attributes with the new AF system, 11fps continuous shooting, and a relatively affordable price, and you have a very enticing camera for sports and action photographers on a budget.

Sony A6400:
USA at BHphoto, Adorama, FocusCamera, Buydig, Amazon.
Europe at Calumet Germany, ParkCameras UK, Jessops.
Asia at Sony Australia and Sony Japan.

Nikon recently stated that high end DSLR sports camera are still superior but this might not be true anymore:

The new Sony A9 5.0 firmware has been tested by Tony Northrup. He now states the A9 AF tracking is better on the Sony A9 than on the Nikon D5. Mirrorless finally did beat DSLR in their last bastion.

And here two more videos:

Sony a9 video tracking fix

Sony a9 Firmware 5.0 Menu walkthrough

Finally Sony learned the leasson from Fuji’s camera support philosophy and released their best firmware update to date. This adds a ton of features and fixes for the A9 making it become a sort of A9II :)
But they key here is the new autofocus system which simply works like a charm. Here are a few examples but please feel free to add your test in the comment system!

And a discussion about the new fw upgrade: