I don't think thunderbolt will displace the use of fibre channel HBAs and mini-SAS at the moment. There are a lot of existing things using mini-SAS with greater bandwidth, and it's a very small connector.
These are storage-only interconnects, Thunderbolt is PCI so has much wider uses. Concerning bandwidth, you still have to have storage that would bottleneck TB. OWC has a product called Jupiter:
For fast high capacity storage, you'd need multiple 15k drives in RAID 0. You wouldn't want more than 4 drives in RAID 0 and even then you'd put them in RAID 10 so you need 8 drives and you'll still only get 800MB/s, which TB can handle. With SSD, you can exceed those speeds but the types of files that need over 1000MB/s use up a lot of space (90 minutes at 300MB/s = 1.6TB) so it makes the solution expensive. $5,000 for the box + 8 x $500 for 2TB of SSD = $9,000. Not many people will be buying a $3,000 MP along with $9,000 of storage.
Even OWC make a Thunderbolt adaptor to allow connection to the SAS and the CEO says:
“With the advanced processors modern MacBook Airs, MacBook Pros, iMacs, and even the Mac mini offer, these machines are very capable of running advanced applications and processes that rival the Mac Pro,” said Larry O’Connor, Founder and CEO, Other World Computing. “Now with the Mercury Helios, users of these non-PCIe slot equipped machines can tap into the power of PCIe cards and experience capabilities previously unattainable.”
NVidia may not ship that many of them, but the margins are most likely quite high.
That's the point though. Apple would ship higher volume so they don't need to make the margins so high.
assuming enough capacity exists to turn out this many, I don't think Apple would be the one to make such a bold move in the workstation market.
This is the same Apple that brought us the personal computer, the iPod, iPhone, iPad, the first usable unix desktop, OpenCL. Yeah, they aren't the type to make bold moves. Yield could be an issue if they aimed for 50-core chips but they don't need to do that. A 24-36 core co-processor would work ok.
I'm still wondering where you saw a late 2013 quote. The only ones I can turn up are quite ambiguous and merely point to 2013 without even truly confirming a mac pro.
Tim Cook: "Our pro customers are really important to us... don't worry as we're working on something really great for later next year."
If he meant early 2013, he'd have just said "next year". If they intended on using Sandy Bridge, they would have used it.