All - Not that there was any doubt, but a FirePro W7000 does work in a 5,1 MP. (Running 10.12.6; it shows up as Radeon HD 7xxx) Only problem is fit - with the bottom plate on, it simply does not fit. And rather than hacksaw part of it, I removed the bottom plate - 9 screws - and now am concerned that it may overheat - it appears that the bottom plate is a heatsink. It's installed in the second PCI slot, with nothing above or below it, and I've set MacsFanControl to run the PCI fan based on ambient temperature to provide more airflow to cool it. Any issues anyone can see? Should I just cut off the tail end of the bottom plate/heatsink? Thoughts appreciated!