Hey another LPE man! Congrats! I take it you are talking the 570 hp package, they do do a fuel pump booster and new injectors as well. I am currntly up there geting the 650 package and asked about headers, they told me that you need a little backpressure and the stock system actually flows very well. They figure the gain with headers is not worth the cost, only reason to do a catback was for sound. Sounds strange to me but they have probably spent more time with the ls3 on a dyno than most so I figure I will take their word for it.
Take a look at this thread to see what other LPE freaks are doing to their cars:
http://www.camaro5.com/forums/showthread.php?t=40101