
Hard to estimate because of the variables involved. It would depend on the RAW decode quality you choose and just how much grading and intensive nodes (such as noise reduction) you use. You could probably set the decode to its lowest (1/16th) and if your grades aren't intensive you would easily get realtime playback on a 2013 mac pro and either of those windows options. But thats probably not what you would be after.
Right now (before a new mac pro is revealed/released) the absolute fastest Mac solution would be an iMac Pro, with the custom Vega 64 option and then one or two additional Vega 64s in EGPU boxes. Something like that **may** well be able to handle what you are proposing. You can get nvidia GPUs to work in egpu boxes but its hacky/not officially supported.