I am skeptical that bitflip errors above the storage layer are as common as the ZFS authors say, and their statistics that I have seen somehow lack a lot of detail about how they were gathered. If, say, a device with 100 errors counts as 100 instances for their statistics..... Well, it would be nice to know how they were gathered. Next time I meet them I must ask.
That said, if users want it, there should be a plugin that checks the bits. I agree that stripe awareness and the need to signal the underlying raid that a block needs to be recovered is important. Checksumming at the fs level seems like a reasonable plugin. I have no opinion on the computational cost of ECC vs. checksums, I will trust that you are correct.
