I read an article in an RC mag a few years ago that said spinners have an effect on prop performance.....don't ask where or when I read it.
But look at it this way...noise is turbulent air.....a spinner would have the effect of causing calmer air at the hub.....I can see a spinner quieting a prop if there wasn't a spinner there in the first place.....but changing to a 1/2 in bigger spinner...would lead me to think it was more of a spinner/prop combination that lead to the quiter run