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.
Edit: Yea, what billyfe390 said!