With the unsupported card, did you delete the text file in the installation folder that lists the supported GPUs? I don’t recall the name (because I have deleted it so that my K4000 card would e recognized), but at least on Windows it has “gpu” in the name, and contains a textual list of supported cards. If you don’t delete this file (or add the GPU name precisely into the list — but deleting the file is better), then Premiere will not use the card.
Note that the supported list isn’t a list of the ponly cards that work — it’s just a list of the cards that Adobe has had time to extensively test. My k4000 (and K2000m) cards work great with Premiere and After Effects, once you delete the supported GPUs file in the case of Premiere, or add the card to After Effect’s card list (deleting the after effects file doesn’t seem to work).
And, you have to do this again after each time Premiere or After Effects gets updated.
Hope that helps!