exactly. You got a deal on it, find out what it sounds like, if you don't like you might even make money on it. Who knows.
As far as the horse power loss, I agree you could have 7 to 10 hp between identical cars, and like Darrell have seen this first hand with the LS Camaros which we referred to the higher hp cars as "factory freak" because of the bonus hp, in the same make and model car. BUT if you "know" your taking a hit on hp, by installing a optional set up I think a lot less buyers would have checked the box for chambered at the time. That said it was many years after I think before it was discovered.
I still run into guys who proudly state they have the chambered exhaust. Which leads m to believe that a lot of guys probably thought(think) it was hp booster over the transverse system.