Why manual and not a dynamic allocation of power budget like gaming laptops use? The driver decides which of the CPU or GPU is "starved" and gives the other more power.
One theory: If a game is GPU limited, which most are, it will be at 100% utilization no matter how much power it steals. However the CPU can't be power limited too much. Games have a physics loop that has to runs at a constant rate, independent of rendering. If the CPU is at 100% any disturbance might cause the physics step to not finish in time, and the game crashes...