tbh you wont notice the difference in parasitic loss in terms of acceleration but you will in terms of engine response and engine braking that is all. We run alloy pulleys on all sorts like smaller diameter pulleys for ancilleries to offset higher revs, for example pas pump maybe ok to 7krpm but were often pulling well over 9k in race cars this caused lots of pump failures, reducing diameter solved this cheaply and lighter was just a bonus really.