Mojo one of the main benifits of multiple subs is more headroom and less strain on the individual subs. Accordingly at any level of SPL you should see an improvement in distortion and group delay. You create extra headroom by co-locating the subs due to coupling. The major drawback is the lack of room smoothing when you have subs located throughout the room. The trick is to have the subs close enough for mutual reinforcement and far enough for room smoothing. Easier said than done.

John