Note that the first comment there is an illumos developer offering to help -- and when Chris tweeted about this issue, he was offered similar help from the community.[1] It would be great to have a better characterization of this issue (which is not as simple as it not working, but rather is apparently a performance pathology that appears under load) -- it's entirely possible (if not likely) that this is (just) a driver bug, and not emblematic of larger issues. Certainly, taken alone it does not indicate that the OS is "struggling" (speaking for ourselves, we're 10G everywhere -- though not 10GBASE-T).

[1] https://twitter.com/thatcks/status/517404451121143809