I am still slightly confused. The original AMD "45% faster with Mantle" slide specifically stated the hardware was Kaveri 7850k and 290x. Does this imply the more powerful the GPU the more load Mantle can offload from the CPU to the GPU and thus improve overall performance?
This is all about reducing the amount of work the CPU must do between the time a render function is called by a program and the time when the GPU takes over to do its job. There's plenty of code must be executed by the CPU before the GPU is ready to work. Mantle reduces this work.
What AMD has figured out is that users would rather not have to upgrade their entire system to take advantage of ever faster GPUs. A user that spends less money on a CPU (especially a high end Intel processor) has more to spend on an AMD GPU.