I've got an '05 WRX. I replaced the stock muffler with a Magnaflow. It's a nice sounding upgrade. I might do a cat back later.
Anyway, when I had the Magnaflow installed the guy said that he could remove one of the two cats for a little louder sound. At the time I said no, but I might do it now if it wouldn't affect my emissions test.
If anyone knows for sure please let me know.