100% true about the performance cliff, though at Delphix we worked a bunch to raise performance before the cliff and to push out that cliff. All COW filesystems filesystems (and I'll throw SSD FTLs into there as well) require over-provisioning for consistent performance.

And it's also a fair point that for enterprise storage running at 80%-90% of capacity is a reasonable restriction whereas the drive on my laptop is always basically %99 full. It will be interesting to see how APFS addresses this since I'd guess that most of their users look more like my laptop than my file server.