Because at some point the filesystem becomes a bottleneck. ZFS was designed with the assumption that CPUs would be way faster than storage. When you get speeds over 10GB/sec, [0] you are going to spend a lot of time checksumming all that data.
Maybe I'm reading those benchmarks wrong, but they appear to max out well under 10GB/s. This would mean you'd be CPU bound on your checksums alone with one of those Seagate cards.
Just one idea: offload checksum calculation to a DMA engine. Linux already has a generic DMA engine facility in the kernel, backed by e.g. I/OAT on some Intel hardware.
Assuming this is the same thing as hardware-assisted checksums, both the ZFS on Linux maintainer and Intel have said that they are working on it at various cons last year.
Because at some point the filesystem becomes a bottleneck. ZFS was designed with the assumption that CPUs would be way faster than storage. When you get speeds over 10GB/sec, [0] you are going to spend a lot of time checksumming all that data.
[0] http://www.seagate.com/ca/en/about-seagate/news/seagate-demo...