I have a 10G Ethernet card on my iMac via an external Thunderbolt PCIe expansion chassis. So, OS X can use 10G.
I have a third party software iSCSI Initiator installed, so OS X can use iSCSI.
You fail to mention NFS, which OS X supports natively.
Aside from CPU upgradability, the 6 Thunderbolt ports means that this particular Mac Pro is the single most expandable Mac on the planet.
You can add a TON of PCIe expansion boxes to give you a ton of slots as needed, including additional graphics cards, RAID cards, etc.
Since everything is external, you also don't need to worry about upgrading the internal PS of the Pro or worry about cooling needs of the computer itself.
Yes, it'll mean that everything is external, but it also means that everything is "pay as you grow" and keeps the computer itself nice and small and hopefully less expensive (we shall see).
No, it's not for everyone, but it is an interesting design and it is expandable.