The last time the sonnet enclosure came up it was determined that the bandwidth of thunderbolt is simply not yet enough for a high end graphics card (especially if we're talking about upgrading years from now) and the enclosure itself did not provide enough power (only 100-150w). Not to mention there are no reports of this actually working with any graphics card, let alone a high end one worth upgrading to.
Just to demonstrate the bandwidth of PCIe3 x16: PCIe1.1 x16, PCIe2 x8 and PCIe3 x4 all provide around 4GBps. That's right, not 4 Gbps. So that's 32Gbps times 4 for a grand total of 128Gbps over PCIe3 x16. In comparison, Thunderbolt 2 at 20Gbps is very constricted.
I'm not sure the external GPU route will ever be useful for professionals, but I'm hopeful that it will become a more viable option for gamers who want a laptop that is light and thin, and the decently portable option to plug in a midrange graphics card that would work way harder than any integrated graphics could.
Sonnet makes an external PCIe Thunderbolt enclosure that you can pop a graphics card into, and there is that DIY guy: http://www.extremetech.com/extreme/162652-diy-external-thund...
Granted that is not pretty and there are OS support issues. But it is possible and I am hopeful it gets easier.