If you haven't noticed, I like stirring up a bit 'o trouble with exhaust discussion. Engine Masters just put up a new video, if you have Motor Trend access, on mandrel-bent versus crunch bent exhaust. They got 2 hp difference on a 540 hp LS3 with a 2.5" exhaust system. The crunch-bent tubing they had was a worst-case job. There was a whole psi of difference in backpressure, but the power was almost within a margin of error. They also tested straight-through mufflers versus Dynomax Super Turbo mufflers on the exhausts and got nearly no difference, but the Dynomaxes were quieter.
They do have a bit of discussion on backpressure, because below 3500 rpm the full exhaust picked up torque compared to the open headers with extensions, but they state they believe that's a result of the length of the pressure wave and not backpressure. I'd love to see a test on that.
I remember back in the '80s when mandrel-bent exhausts became "the new big thing". People were getting amazing horsepower gains by ditching the factory crunch-bent pipes and putting on big mandrel-bent tubes. We were all sold on mandrel-bent tubing, but the infamous "header bash" episode of Engine Masters made me do some re-thinking, and for a while now I've believed the problem was less the crunch bends, and more the tiny tubing the manufacturers were using in the '80s and '90s. My '99 Suburban had 1 7/8" crunch-bent tubing behind the manifolds. It would have been small on a 305 TBI with a factory cam. Putting any larger tubing in front of it was going to be a positive change.
The one positive thing about mandrel bends is that you can buy pieces ready-to-go fairly cheap. If I'd taken my truck to the exhaust shop and had them bend tubing for me, I would have been out more money than buying pre-bent tubes and welding it myself.
I am thinking about replacing my Cherry Bomb Salutes with some Dynomaxes now.