There is no claimed performance boost, they just talked about believing there is a certain improvement, abstractly. Some gullible people heard the "900% faster draw calls" and thought it means 900% faster performance but even AMD would deny that, easily. In all likelihood Mantle would offer between 0% or worse performance difference (if the programmers are incompetent) and around 2% for good basic usage with some rare programmer Wizards possibly achieving more (4%? 10%?), because we are not in the 1990s anymore, general purpose APIs are already not involving the CPU/main system extensively hence you have to know what you are doing, not just using it for the sake of using it.