Tampering with your cats will make your WRX illegal no questions asked. However, the purpose of putting a highflow cat is to pass emmisions. That doesn't nessarrly mean they check to see if any are missing or swapped. Most places don't care or even take all the effort as long as you're not in the California area. Most places as long as it passes a safe level, you're free to go. Personally here, they don't even use the sniffer either.
"We live for a good time, not a long time."
RIP Colin McRae