You could find such posts, ones that also say the performance difference 6 months from now in DX12 titles will be a different story. DX12 will make such utilising new features a far far quicker process than happened for the 7970/290x.
Also yes, the RX480 is tuned for tdp efficiency rather than performance. What is to be seen is if it can overclock well with a AIB card tuned for power delivery and clockspeed while using higher power.
Regardless I think a year from now in DX12 games the RX480 will be beating the 390x with ease in basically every game, 6 months, beating it in most games, a few months matching or beating it.