Bouncy rubbers are not always faster, a softer one (within reason) will be bouncier when you drop a ball on it; however, at higher impact (think, loopdrive/smash) the softer rubber rubber will bottom out and will produce less speed than a harder rubber. Another factor that can influence this is tackiness of the rubber (Chinese rubbers tend to be bad for smashing), but the ones you're talking about aren't really tacky enough to have an effect.