Significantly faster? I don't know- I think what resolution DOC's gaming at is the determining factor, and what is the minimal acceptable framerate in his opinion.
I'd venture a single GTX285 is plenty for 1680x1050. But I'm betting he games at 1920x1200... so that might be a bit much for a single 285. Thus, running SLi will allow him to have sustained framerates that are acceptable, and the one card with the lightest work load just does PhysX.
Kind of a hard call, not having tinkered with such myself.
I'm under the impression though that a single GTX285 is over kill for dedicated physX- no game out can fully use the card just for PhysX. Hence why keeping SLi and just enabling GPU accelerated PhysX is probably the best option... all just a guess though.