I have a Borla Off-Road X-Pipe. The common price I can find it for online is around $700... but I got lucky and bought mine off a guy I met at the dealership used for $100 cash. It does make a decent difference in the power, but the sound is a bit raspy. If you can find one for a few hundred bucks, go for it, otherwise it's not worth $700. And I don't have O2 simulators, haha, I just deal with the check engine light. Doesn't affect anything.
I'm considering an m50 manifold though, as it's supposted to help my top end greatly and would be a nice pair to install with my UUC Pulleys (still haven't gotten them on yet), since they increase low end.