Hello!
( NOTE: I am not an Epic-developer, and I do not claim to be one. I try to help you as a private person)
Both of the cards featured as “compatible” are extremely old, hence your trouble finding information about them.
I am not too sure about MAC-compatability, as generally mac is not upgraded (for obvious reasons).
You may want to look into (from AMD side) R9-series, find your price range (they vary from 200$-700$, + extreme “R9 295x2” and check compatability issues, and (hopefully) make a purchase. If MAC is not compatible with AMD ´cards, you may want to have someone who knows about them inform you.
Hope this helps, -DoctorPC