You mean crush bent? I don't think it'll make THAT big of a difference, but if I put it on a dyno I'm sure it would show. On some cars it really doesn't matter if it's crushed or mandrel because the bends aren't that bad. I'm thinking on our cars they are. I'm going with Mandrel myself.
actually, crush bent is different from compression bent. crush bent has little ripples in the inside of the curve, while compression bent looks smooth, just with an indentation, looking like a smaller pipe was used for the curved part. mandrel bent is by far the best, because the exhaust is a gas, and a gas flows better through an unrestricted curve than a bumpy one. also, the smoothness of mandrel bending allows the gas to gain velocity that the other type of bends dont allow. there is a word for this, but i cant remember right now. the gas basically starts to spiral, and gain speed towards the end of the pipe. this effectively helps exhaust scavenging.
p.s. this spiraling of air can also help on the intake side as well!! to cram more air into the intake