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They're not going to be using DX12 for the Frostbite Engine in ME:A at launch, but there is support already built into it. Whether they will make the effort to port it over though is another matter.
With DirectX 12 it's tough enough just to match DirectX 11 performance so they may judge it not worth their while for now.
That said, at the moment the 8GB RX 480 is a great price and should nail ME:A at 1080p settings at the very least.