For someone who runs Crossfire R9 390x (three cards) DX12 makes no difference in term of performance. For all BS Windows 10 brings not worth *downgrading to considering that lot of games under Windows 10 are simply broken or run like garbage where no issue under Windows 7.
There are no DX12 games yet for review, so why would you assume that you should see better performance in games made for windows 7 DX11, in win10 DX12? Especially in "tri-Fire". DX12 has significant advantages over DX11 so you should wait till these games actually come out before making assumptions on performance, or the validity of DX12's ability to increase performance.
My games, FO4, GTAV and others run better in Win10 and i have had zero problems. I think your issue is more Driver related, which is on AMD's side, not MS's operating system.
I'm on the Red team by the way.