is the PHX-D20HDSDI in x4 PCI Express Form Factor" in a Sonnet "Echo Express III-R Rackmount Thunderbolt 2 Expansion Chassis" still the best choice?
http://www.sonnettech.com/product/echoexpress3r.html

Also has anyone played with the Sonnet Mobile rack kit? Looks like it will add 8 drives to the 2U expansion card chasse. http://www.sonnettech.com/product/mobilerackkit.html