Just to post an update to my earlier posts on the RX460... short version: it's going back. I am not contradicting anyone else for whom it's worked out, good luck to you, but in my particular circumstances it wasn't going to work. I've had sleep issues with a 5770 alongside it (didn't wake sometimes) and then full-blast fans when I've tried waking it alongside a GT-120 instead (every time... so that is not going to work for me). If you're not using filevault so don't need a bootscreen it's probably a fine card.
In other news, the row of locking attached to the latch on the back of my Mac Pro decided to snap😱, internally😵, right when I went to replace the PCIe holding screw panel I forgot to put back as I putting everything back to how I had it, so this has been an interesting evening of hammers and prising very solid aluminium open! 😳 I have a new, less elegant external latch mechanism now, made by Gorilla Tape. 😉