I suppose on a dyno or a drag strip the mandrel bent exhaust can make some difference.. I never could really feel that much difference though..
AS far as no name mufflers, a little examination is required, but I'm finding that the no name knock offs to be of equal quality to the name brand less the cost of the stickers that come with them.. Once again a little examination is required, a good look at the construction, and materials used can usually tell you if it's any good or not..
AS far as stainless goes, I'm not up to speed on what all the grades are, but there is some really crappy stainless out there.. I dunno maybe it's a stainless coated pipe or something?
IN this neck of the woods we get our fair share of salt and slop yet I'm finding that the aluminized pipe is holding up nicely.. They coated the welds with some sort of crap and so far not even any noticeable rust there after 5 years.. For a 3rd of the cost of stainless pipe I went with it and fully expect the car to rust apart before I have to put 3 exhaust systems on it..