The reason cats don't sail with symmetrics (or any other lighter boats) is two fold:

1. catamarans accelerate so quickly that the apparent wind moves forward quite a bit making you almost always sailing a reach. Even when sailing deep downwind, you will find your apparent is actually at 60-80 degrees off the bow. An Asymetric is cut for sailing these angles closer to the wind. Most symettrics are designed to be sailing with apparent wind angles of 90 degrees or more.

2. The other benefit of a asym on a sprit is that the sail creates more lift on the bows (longer bow sprit=more lift). Catamarans like alot of lift on the bows (helps counteract the pitching moment from the top of the main).

You combine these two conditions and it becomes quite clear that the only sail to get is an Asym. You need to resist the temptation to pick up a cheap symmetrical because in the end it would essentially be a waste of money.

Good Luck!!