That's bullshit. GCN 1.2 supports Mantle. And no need to do more. Of course it don't able to use some specific features, but GCN 1.2 will be as good as GCN 1.1.
With these explicit APIs the driver is not really more than a shader complier. This means all the things that was in the kernel driver now must be implemented in the application code. This will eliminate the kernel driver and all of it's server threads permanently, which helps to use the CPU more efficiently. As a drawback upgrading the driver won't do give any advantage, because the application has explicit control over the memory management, and the implemented optimizations might not works on the newer architectures as efficient as on the older ones. That's why Battlefield 4 doesn't works well on GCN 1.2, because it never get patched to support that architecture efficiently. But the never games are optimized for GCN 1.2 - out of box or with a patch.
This is exactly the same model what D3D12 and Vulkan will introduce, and it's really sad that nobody say anything about it's drawbacks. Don't get me wrong, I think these APIs will allow to create more optimized PC ports, but sure getting a new hardware that doesn't really supported on the actual games may result equal or worse performance that it's predecessor, even if the new hardware is faster in theory. The developers have to make a patch with some specific optimizations. There are several interesting strategies to make this less problematic, but on the PC it will need some bigger engine changes, so this is an ongoing research and it won't have results in the short term.