As you know, it's a big problem that having 60 registered members online, and 687 guests viewing virtually the same content. It leaves little incentive to actually register. Where as, if we were to limit the content they had access to as well as promote the sections they can't see heavily within the sections they can see, it could tip the guests that were debating whether to sign up, to actually sign up out of curiosity for the magic behind door number 3!
Possibly could have the opposite effect of course, but could be worth a trial.