Yeah, it increases rasp a lot. How badly depends on the exhaust you're running now (stock vs straight pipe, etc). A good setup is the mvp header + Blitz front pipe + Blitz catback. Yeah it's loud enough to get tickets, but you'll sound a LOT like a modded TT supra.
I'll agree with everyone else. The header doesn't really add any power unless you like to drive above 4000rpm a lot. It hurts daily driving, especially if you're auto.