What's wrong with having an annoying exhaust on an auto?
You're not going to get any HP gains on an NA motor, so there's no point anyway besides obnoxious noise.
But I don't get what the difference is on an auto vs. manual in this regard. It might not sound exactly the same when the transmission shifts vs shifting a manual. But it's the same engine making the same noise, regardless what transmission it's hooked up to.
I've had an auto 2.2 car, and a manual 2.2 car. On both cars I've had the muffler rust off. They both sonuded more or less the same...