a GTX 670/760 2gb from ebay may be worth a look then there cheep and almost as fast as a gtx 680 + work with osx native drivers & there power levels are safe (2x 6 pin) a 670/760 4GB might also be ok price.
the 670 is faster and cheaper than the 760 but a tad older and there fairly close.
this is the thread about the new ati cards and there's a thread about the new nvida ones to but simply they dont work at the mo in osx and we dont know when they will, there's is the indication that the next version of osx will have drivers for them but we dont know 100% yet and for nvidia we will not know if they ever will work until nvidia give us drivers for them (if they ever do)
https://forums.macrumors.com/thread...-usd-other-apu-polaris-announcements.1975249/
edit when you say playback is choppy when playing 4K is that when the files are uncompressed or compressed, if it's chopy when playing prorez or dnx then it's more likely to be drive speed than the gpu but i may be wrong
ps what app is chopy when playing the files?
try putting a 4K file on the SSD and playing it see if play back is smooth
edit
it's much harder to play h264 than prorez on the cpu so if you can play h264 it indcates drive speed being the problem look at adobe's forums for and see all the complaints about h264 playback in premier (unless thats been fixed in newer updates of CC)
https://www.google.co.uk/search?q=p...v8Aff3rqoBQ#q=premiere+h264+playback+problems
at the mo i have a gtx 660 i have my os on a SSD and i use a 1TB WD black for my live media file and a WD blue for my scratch drive and a second 1TB WD black to export too which tends to work well for light video editing 1080p most my video is h264 now from my canon 6D.
only when using FCX or resolve do i see my GPU working hard.
saying all that if i was you id upgrade that cards fairly old now, i got my gtx 660 back when i got my 3.1 to replace the GT120 and relay enjoyed the upgrade ^^
for premier this is worth a look
https://blogs.adobe.com/creativecloud/red-yellow-and-green-render-bars/?segment=dva