I've never sailed a 5.7 but I've seen one a while back. It's an interesting catamaran with a skeg keel instead of daggerboards. Because of this, it's upwind performance will be slightly less than a boat with daggerboards...however, when it comes to sailing in shallow water or beaching the boat, it's one less thing to deal with since daggerboards must be raised and lowered. I imagine the boat is a bit heavier than the 5.5. While any boat can be singlehanded, I would say that it's getting a little beyond what you might want to consider a typical singlehander size-wise.

If you do plan to race, you may definitely want to consider what kind of boats you will be racing against. Most regattas have an open class that will group all sorts of onesies and twosies catamarans and adjust finish times using a national handicap system. This kind of racing is fine for some but most of the best learning experience can be found if you can race within a number of identical boats. The majority of serious racers prefer to race this way because the playing field is so much more even. I would certainly check around with the local catamaran clubs and see what is being raced locally if you think that might be a consideration. As it's winter most everywhere in the US, there may not be a whole lot of action at the moment but you may even consider hitching a ride during a few regattas to get a feel for what you do and don't like.