I am curious why RAID6 is (still) used given its poor behavior in failure conditions. I am guessing that Backblaze is not as worried about additional drive failure in an array during a rebuild because they are periodically verifying each drive.
Comparing to a X4540 is kind of a stretch. The use cases seem very different. Also, it for Backblaze, they wouldn't exist at the price they charge, with the price that Sun charged for an X4540.
I hope Joyent does release something about their config, that would be very interesting.