On my particular Win7 machine, DX12 runs noticeably worse than DX11 Legacy, but the DX11 multithreaded version runs very well indeed.
DX12 gives me lots of microstutters, likely due to a similar bug to the one that crops up for Mac users occasionally, where textures are streamed from the disk every time they’re needed instead of cached in vram like they should be. But that’s just speculation on my part. The api was ported to a system it wasn’t designed for, so even with latest graphics drivers there’s bound to be some hiccups when using it.
It’s still worth the effort to upgrade to 7, just don’t expect performance to go through the roof by using DX12 instead of 11.