I wanted to close this thread as I so hate coming across a thread that doesn’t finalize with an answer. We installed a Sapphire HD7950 video card in the Mid-2012 machine and tested. It eliminated the freezing that is so common on the mac when using UE4. The vehicle demo, which was unusable works well, graphics are smooth and no freezes. The high cost of this card ($479) makes it questionable if it’s worth staying with the mac for the development system unless one has another reason to do so. We do, so it’s a good solution.