Just got the time to look at the file randomness. Anvil, can you run a CPU benchmark of whatever generates that random file?
That's the reason I asked for it, it looks like a hash of random bits rather than random bits.

The reason I ask is if this can't do something like 500MB/s for generation, and it's done in the same thread as the writing, you are essentially doing sequential transfers mostly (the overall "write" speed would be an indicator)
Regardless of how random the file is (compression-wise) or your internal file I/O, it seems the overall I/O is mostly sequential.

Though I am not sure I understand any more what you guys are trying to achieve, but with such small random I/O, it is hardly real-world.