Obviously any advanced metrics on the Steamboat are skewed due to inconsistent minutes, but according to basketball-reference, his defensive rating is currently a 91, which is pretty [dang]ed good. If he maintained that for an entire season of regular minutes, that would be a top 25 all time (KG's best defensive rating for a season was 91.61).
Now, don't misquote me, here, I am not saying that he is THAT good. His numbers are skewed because he doesn't get many minutes, and he hasn't really been forced to match up with a good offensive big man (which are so rare in the NBA anyway these days). So while you would expect that number to fall if he got, say, KG's minutes, in all likelihood it would still stay pretty elite.
(Dwight Howard is currently 93 in defensive rating, and is considered 1st in the league because Steamers hasn't played enough minutes, in case anyone is wondering)