There is no power gain but there will be a minute ET gain since the power isnt wasted in the motor turning in the mounts instead it will put more into the drivetrain.
Don't harder motor mount bushings mean less loss of power? Not a gain but a reduction in loss. Less energy is spent and absorbed by flexing of the car. That means more energy is sent to the wheels. I realize we are talking miniscule amounts here but isn't that the theory behind solid motor mounts? I guess anything harder than the stock spongy bushings would make a difference, though probably measurable in a lab environment only.