Right now DX11 MT is good, but it's not nearly on the same level as Mantle. DX12 MT should be on the same level as Mantle from a performance perspective (or a bit better perhaps as there are many other improvements across the board than just MT implementation) but without the drawback of being tied to GCN..
Oh but judging by NV proudly waving their charts and pledging Fermi-deep DX12 support,
and AMD being awfully quite about the same issue, it does seem like DX12 will be tied to GCN.
From AMD's perspective that is.