Actually the answer to your last question was already answered (Yes). The spinnaker provides lift down wind actually making it safer (by not pitchpoling). Down wind the forces on the main will push the bows down until wind reaches the point that it turns over the boat. The spinnaker helps balance the forces so, some force is in front of mast and some behind mast, result boat rides more balanced. Also confirming, total waste of money to even consider symetrical spin. Don't even think about trying to have one cut down. Even the newer monohulls have no use for them, except in extreme conditions. For a catamaran it is the same as asking, "I am a guy, should I wear a bikini or shorts for a bathing suit?" It is just wrong!
Even a Hobie 14 will improve dramatically using a spinnaker to reach down wind. Also if you are trying to "fit" the wrong size on the boat, smaller/flatter is better than too big. Too big will be bad when trying to create apparent wind. Have fun and go for it.