don't forget that even the macbook air with HD4000 can do that for 27" thunderbolt display...so i don't think there are problems with the technologies.
Like apple said with GPU+thunderbolt 2 of MAC PRO you can have 3x4K.So i guess with actual thunderbolt we can deliver 4K or retina 27" displays. Probably just 1 display but it is enough for now.
the processing power of the GPU is more than enough to drive 4K. It just depends on whether they built the proper graphics IO into the chips. When Apple is pushing it, Intel/AMD/NVIDIA surely will deliver