I have heard different claims about the Steeda motor mounts but have not seen any dyno results so far. The idea is that they lessen the amount of power lost to engine vibration and therefore give you a few extra HP. Here is what Steeda says about them:
'The weakest link in your drivetrain may be the hydra-mounts used for mounting the engine. While they get the damping job done, they can be prone to fatigue and/or leaking under hard usage. These direct replacements eliminate the hydra-coupling, and do a better job of directing your engine's efforts to the wheels."