stuff like this (the op post, not threadjackers) that makes me never EVER want to buy a used car. the balance of the engine is for longevity purposes. if there is less vibration, there will be less internal complications due to the vibrations that could occur. this would mostly involve the life of the crankshaft bearings.
and since this makes it seem like all the balancing is internal, this might make it a bit easier to change out something that would make a much bigger difference, like the clutch components, mainly the flywheel. if the flywheel is lighter it will free up hp mostly in the first gears, as well as free-revving faster. and faster revving = better shift quality during quicker shifting applications