Not only can they restrict some power but they make the exhaust sound wierd, I don't like it. The worst part about those things though is when racing, an "all thumbs" marshall can very easily grad the sides of the vehicle and crush the rubber stinger, killing the engine. This is just as easily done by crashing against a pipe. Not good.

BTW- I have never hurt my thumb by bumping the flywheel. You just have to make sure your orientation is right. The flywheel, when looking through the bottom of the chassis, is spinning towards the front. This means if you touch your thumb to it with the tip of your thumb pointing to the front, you'll be alright. If you point it to the back, the flywheel will bite and grab on to you, that's what hurts. It's just like stubbing your toe on pavement.