An update for the guys with the same question, I have not heard anything on the 390x but I did get confirmation from AMD on facebook that the Fury will indeed support the mixed rotation eyefinity.
I don't know about any further driver updates or what will happen going forward but as of 15.7, the Fury x does not support mixed rotation eyefinity as far as I can see, leading me to believe that the 300 series doesn't either. I do think they will address this in a future driver update however.
I have a a 3x2 setup and cannot for the life of me get the top row of monitors to rotate, they are either all portrait or all landscape and all the same orientation. I have tried everything that others on the forums have suggested but to no avail. I have decided just to flip around my top monitors and wait for another update to hopefully correct this.
I hope this helps.
I would also like to ask this question. I own sapphire 390x and wish to have eyefinity plp working on it. surely this is something that can be easily done via Crimson?