I'd love ZFS a lot more if it weren't for the ARC. Using ARC means that ZFS caches stuff outside the kernel page cache. That in turn means that sendfile is no longer zero-copy when data originates on ZFS, making it a non-starter for content drives for our CDN workload.

Yes, but this is a Linux/licensing problem. Use one of the BSDs and the ARC doesn't fight with the system cache.