I don't think that's how it works. There might be some minor improvements as far as the OS using the new API is concerned but I wouldn't expect for the games themselves to change. For example DX11 bought performance improvements over DX9 & DX10 but that didn't mean games made for the two older APIs ran better if the new one was on the system. Also, Mantle might be a different API but it uses the same principle as DX12.
WARNING! (car analogy): Let's say there's a dirt road and a highway. The driver only knows about the dirt road so that is the only one that he'll use (unless somebody tells him about the highway).
What I'm saying is that, unless the old games themselves get patched to use the DX12, there's not going to be any magical performance boost. That Star Swarm that everybody is using was built specifically for that lower level API access. I would have expected some older DX11 demos to be upgraded for Mantle/DX12 by now if your claim were feasible.
I said already that you'll get the most out of it if you write to it specifically but I expect a boost in performance regardless, DX12 can still handle DX11 code, that's why it's called a superset, there's a level of compatability already present. There's also the fact that we're getting driver improvements here, not just API. Everyone knows by now that any driver performance gains show up for games without those needing to get patched.