I don't think external GPUs are part of Apple's plan. The GPU comes inside the Mac, and if you want to upgrade it you can buy a new Mac.
Lately, I haven't felt like Apple has my back for pro-A/V :/
What Apple has designed could be described as a laptop for your desk. Sales of systems where the CPU/GPU are upgradable are plummeting. The market is driving computers in this direction.
I don't necessarily want a top end CPU, but I like a decent GPU and a lot of memory.
Not to mention the environmental factors.
OS X system guides - http://www.tonymacx86.com/