@destraudo well said! That's an excellent description of data transfer bandwidth.
@TheSwanCollective, all three units (5K, 8K, and 8KX) use SOME of the same chips.
There's a chip that handles the DisplayPort 1.4 signal from the GPU (actually, the 8KX has 2 of those) and the 8K also has a scaler chip, which stretches the 1440p input resolution to the 4K native LCD resolution, but the other 2 headsets do not include that chip.
It's not 100% clear, but it appears that the scaler chip is the source of the 80 Hz refresh limit for the 8K unit. The 5K doesn't have that chip, so it can run at 90 Hz. It's likely that the 8KX will also be able to run at 90 Hz, since it doesn't have a scaler chip either.