I can't read any of the links at work besides the Fudzilla one, but that offers zero proof of any performance gains. It speculates how Mantle will work and even says "along side OpenGL and DirectX" which means it will have very little, if any, performance gains. The majority of DX slowdowns is from the DX stack, which can't be bypassed (except for tessellation). Mantle, if it uses DX in any way, will get very limited gains. If it doesn't use DX, it will either have to provide the higher level API functions to emulate everything DX is doing OR rely on the developer to write their own. That means it has the potential for some (read as Carmack and Lottes) to get better performance and everyone else to stick with what they know because entry level programmers can use DX.
So, unless you have a real link to how Mantle actually works and not some "omg it will be so fast because it is more metal than Metallica", everything is speculation.