Please quote a source, wikpedia is not accurate for any form of information.
Here is a block diagram from a
Intel motherboard with Thunderbolt.
[ Anadtech ran an article covering this another board.
http://www.anandtech.com/show/5884/...s-part-2-intels-dz77rek75-asus-p8z77v-premium ]
The Thunderbolt controller is hooked to the PCH. This notion of direct coupling is goofy since Sandy Bridge/Ivy Bridge CPUs don't even have graphics direct output. There is not direct coupling to the CPU at all.
Likewise the Tomshardware article pointed to earlier in the post you are trying to knock down also has a diagram which probably more so illustrates the delusion flaw in your claims about direct CPU coupling.
[ Contextual page.
http://www.tomshardware.com/reviews/thunderbolt-performance-z77a-gd80,3205.html containing the borrowed from Intel graphics. ]
The first option above is all that is needed to make it work in a Mac Pro. The dGPX does not need to be implemented on a detachable PCI-e card. The iMac and MBP 15" models do did it in the 2011 & 2012 without one.
There is also no requirement that the TB controller pick up a display port output from the PCH if there isn't one or that there even be two Display port inputs (some TB controllers don't even have more than 1 input.). The PCH of a new Mac Pro could either use x4 PCI-e from CPU or x4 from its PCH. There is no requirement only to use one or the other.
There are limitations in placing the Thunderbolt connector. It pragmatically needs to be only a couple of inches from the connector ( the very high bit rates relative to even PCI-e make it a problem to place elsewhere).
Pragmatically though there is another limitation. Namely the Thunderbolt controller needs some power and firmware connectivity
[ Again the Thunderbolt 103 section in same article
http://www.tomshardware.com/reviews/thunderbolt-performance-z77a-gd80,3205-4.html ]
The "native host interface" (NHI ) + Thunderbolt Target mode (on Macs ) + the power distribution + "host mode configuration at boot"requirement means these 'custom' host interface on ASUS (and other ) designs for TB cards are more than just a simple DisplayPort connector from the motherboard's PCH DP output.
The block schematics are more about movement of data than of boot/firmare/power specifics. I think that is the tipping point against "plug in cards" for legacy oriented design around PCI-e cards. Not the PCI-e and DisplayPort data traces.
Impossible means can't be done at all; even as a kludge. That is fundamentally different from improbable ( extremely not likely). A kludge that hooks up legacy PCs (including previous Mac Pros ) could be done. As I pointed out before they'd have to kludge around the missing firmware/power/boot issues. It just isn't very likely. It is far more cleaner and easier to pass TB certification to run embedded GPU output along with the other connections needed from the motherboard.