I have yet to meet a manufacturer that doesn't claim gains for any product they make that is "supposed" to add power. If the pullies add say 3hp, that's a gain that will never be felt, and something like pullies is highly subject to something called "diminishing returns" -- on a stock Si they might help more than on a fully bolted on Si. And on a forced induction Si they will have zero benefit -- the motor makes so much power that a weight change in the crank pulley is just irrelevant, you'll never notice in any realistic application (dyno is not a realistic application, sorry).
If you want to talk about reducing rotating mass -- look at the oil pump with balance shafts. I have one sitting on the shelf, it weighs like 20lbs or something (it feels like a boat anchor compared to the RSX oil pump which weighs like 1 or 2 lbs).