You don't need so much bandwidth in real life. I run a GTX970 with a macbook pro 2015 , I get 95% of the perf of a windows machine with a card installed internally. ( and I'm sure that is just because the driver are not so well optimised on MacOSX )
This thing changed my life , first , the macbook pro is not able to drive a 4K screen , in theory yes, but in real life the computer become so hot that the cpu and the gpu are slowed down... second in premiere and resolve you can use the card to do the calculation , when you are working with 4K file this is a life changer !
After I have to say that Bizon are just thief , it's an overpriced shit , just buy the new Akitio Thunderbold 3 , a good graphic card and a Dell 200w power adaptater. Even the software is not from them, they sell with an opensource script that handle the installation of the drivers !
The trick is to convert your files to prores ( instead of h264 ) , and if you want to edit 4K, convert them in 1080p for editing and use the 4K files for rendering only.
For exemple my macbook pro 15" i7 last generation as not enough power for editing h264 files in 4K.
You have to think about buying a fast external drive (USB3 if the mac is supporting it or thunderbolt for older ones ).
The trick is to convert your files to an optimized/proxy media for editing , for example in 1080p. Make the edit in 1080p and after switch back to the 4K files for rendering. It's quite easy to do that with FCPX or Premiere
My MacBook Pro ( 15" , last generation i7 , the biggest one on the market ) is not enough powerful to have a fluid workflow in 4K ;)