I have a m395x 8gb on a laptop right now, so I might have to wait until vp14/15 becomes more suited for the newer-gen GPUs. I don't think it's much of an upgrade going from m395x 8gb to rx580, but what do you think?
I work mostly with PRO Res or Sony Xvac and find the 480 with 8 GB works well for 4K. Also if you have QSV on your intel processor and use the Magix ACC/AVC codec it works well for rendering (speed test here)