True, they will do more for a modded engine than for a stock one. But I've seen guys with a decent setup lose a few tenths with just the addition of longtubes. Depends on the combo and how choked up your exhaust already is.
Do it for sound, not for power. And if you have to pass emissions, or live in a cold climate, think twice. They tend to make the engine run richer, especially when it's chilly outside.