This page is a link that shows how different mufflers compare both in trap speed, quarter mile time, and decibel level. It's interesting to see that a really quiet setup will only cost a little more than a tenth of a second. For a track only car, this is a moot point, but for a street car where one needs to balance sound level and quickness, this article has some good pointers.

http://www.pontiacstreetperformance.com/psp/exhaust.html