Not really, the OS still has to abstract it onto the harddisk, this should be trivial to achieve without impacting page cache and zero copy optimizations if the on-disk metadata is seperated sufficiently. COW Filesystems already do it to a lesser degree and they're certainly fine.
What to do with clients who want a stream of records? Separated meta + payload allows fast byte streams. Combined meta + payload allows fast record streams. Structured implementation can't provide performance for both cases. But byte-oriented can. And we see this approach in modern and fast OSes.
May be I was unclear. If we introduce record abstraction for file ops then OS low-level implementation have to decide which way to store structured data. meta + payload or meta separated from payload. In latter case there is no way to provide fast record streams. Former case does not allow fast byte streams. Client decision to receive bytes or records doesn't magically zero copy reorder stored data.
Why would the later cause record streams to be slow? Snapshots in COW FS' aren't that terribly slow either, even if there is quite a few of them, which basically boils down to the same structure, really.
What a performance-wise nightmare. Goodbye page cache and other zero copy optimizations.