If Mantle really is that big of an improvement, and if Nvidia rolls out their own implementation, we might be inclined to support such a code path.
However, at the moment, this is not a good idea. We have no idea what those extensions are going to do, we have absolutely no clue how they're going to be supported on Linux and MacOS (My guess? MacOS won't see these things, ever). Utilizing vendor-specific functionality is something we can only do if there are significant benefits to doing so, and if the variant code paths are reasonably easy to write and maintain.