Bah, 256 bits is already so vanishingly small it's hard to comprehend. If you put a thousand disks into a pool you might reach 2^40 blocks, leaving you with 256-80=176 bits of margin. That is never going to collide. You could make such a filesystem for each atom on Earth and the odds of a single collision would be less than 0.1% You could put a billion disks in a pool and still have 136 bits of margin.

512 bit hashing is basically placebo.

I agree that 256 bit hashing is totally sufficient. I wouldn't call 512 bits "placebo," though. Just overkill.

You hit a point where the longer runtime increases your chance of error by a very small amount that is nevertheless larger than the protection you gain.