The idea of DX12 is similar to Mantle... not saying copy/paste of the solution but is quite similar. MS finally realized and admitted that there is something except of hardware that can influence gaming performance. I think its still to early to clearly say will or not DX12 will influence games that rely on earlier versions, it might indeed depend of the game optimizations or it might not as some parts of earlier DX components could simply be rewritten and the boost will be instant.
I believe we will know more if the final solution is there and some benchmarks are done.