I'm admittedly not an expert, but isn't that a very different scenario? In other words, ZFS operates on blocks at a block level - it can't make any assumptions

I'm admittedly not an expert, but isn't that a very different scenario? In other words, ZFS operates on blocks at a block level - it can't make any assumptions about the contents other than "this block is the same as this block" or "this block is not the same as this block." It can't (based on my understanding) realize that "this block is almost the same as this block" and shift things accordingly, because then it would be messing with application data that might be corrupted by the changes. If the application is making those shifts on the other hand, it would work just fine because the application could be smart enough to shuffle and unshuffle things in the most efficient way it knows how, but the file system doesn't have the luxury. If I'm still missing something, please let me know. Anyway, it'd be pretty straightforward to properly test this so maybe I'll do that.