recirc o2 housings are usually tubular housing for people who dont want the loudness of a open dump pipe but the performance of the tubular. dump tube in the exhaust is the same princible, just further down in the exhaust, alot of times it will cause turbulance because of the entry of the gases into the exhaust. with a more free flowing o2 housing, you will be able to hear the wastegate alot more, on some of my o2housings ive ran with open downpipe its almost too loud when they open.