Just bought a preowned SI (from an out of town dealership) and it has aftermarket parts.. how does this affect my cars performance?

I recently bought a 2008 civic SI from an out of town dealer so I didnt do a test drive before buying it. The salesman I was dealing with said it was all stock.. the minute I drove it off the lot I knew it wasnt stock because ive already had a stock SI before buying this one. Well I pull off so my boyfriend(yes im a girl) can see whats under the hood. He tells me that theres an aftermarket exhaust, header, high flow cat, and a shifter stabilizer on it! I had no idea and glad the salesman didnt either or else I probably wouldnt have been able to make him come down on the car price as much as I did. But my question is I know what these parts are individually but all together performance wise what all is it doing? And just to note she does drive beautifully picks up quick and smooth. But I'd like to know exactly whats going on under my SI