I'm in the same situation...I live a few minutes away from you as I am in Garden Grove...however, I'm no help because I also have a stock exhaust...my problem is that I don't go to any car events/shows, drag strips, etc. to hear the lovely thunder that an aftermarket exhaust can bring. There's always Cars and Coffee in Irvine Cars and Coffee™ Irvine
I hear a lot of Mustangs hang out there.
You can also visit Hillbank Motorsports in Irvine and check out Ashton's Mustang...He has a Magnaflow setup w/ headers, etc. and it sounds like a freakin jet: Hillbank Motorsports 2012 Ford Mustang GT | Vortech Superchargers Hillbank Motor Sports - Mustang Parts & Mustang Performance Parts