At ~500HP a pair of 3" race mufflers WILL NOT hurt your power noticably. I like the Dynomax Ultraflows. They're still pretty loud (especially with dumps), but not as obnoxious as open headers. That's what I run at the track. For street cruising, I bolt up my Dynomax Super Turbo mufflers. Am I losing peak horsepower? You bet! But how often do I need 600+ HP on the street?
To me, a car with open headers at the track better be running single digit 1/4 mile times... otherwise it's just lame. My local track has been pushing for every car to run mufflers, even the rails. I personally like the idea.