I might be wrong but Mandle seems a replacement for direct3d/opengl and not opencl(/cuda) as one usually need for GPGPU stuff like coin hashing. If so it would be like using 3d gfx api for mining. So I doubt Mantle will change much.
we don't need to use OpenCL for hashinh, any language can do the job, at least most of them...
mantle has way, way 'less loss' in in cycles, memory bandwitdh and tons of other stuffs that other apis still have today.
so yest, it is possible to use new mantle, but as he sayd, some one need to write down a miner that uses that, as when is finished, we should get the 'is it better or not for mining bitcoin/altcoins' but i think will be for sure...
Sorry for long quote!