Mandrel bends are made on a rotary draw bender. Adding a mandrel and wiper die improves tolerances and/or allows tighter CLR bends [/pedantic]
There are a LOT of variables that effect bend quality, 2mm out of round on a 45° bend is pretty poor but i've seen worse* from some sources. Hygienic bends from a stainless stockholder are typically much better quality than exhaust suppliers and usually a chunk cheaper too. The only exception off the top of my head is Steve (Langy) who'll be nicer to deal with, especially if you only want a handfull of bends
Even top quality bends won't be perfect, especially once sectioned even if cut precisely square to the CLR. You just tweak them back with a vice and/or hammer & dolly etc
* http://www.mig-welding.co.uk/forum/threads/welding-304.34462/#post-379248 That's something i did for a good mate of mine, initially he bought a load of bends off the bay and i made him back most of them... deep scratches from worn tooling, humps at the bend tangent from a mandrel nose set too deeply/incorrect pressure die settings etc. They were more expensive too.
The only exception off the top of my head is Steve (Langy) who'll be nicer to deal with, especially if you only want a handfull of bends.
Do I read this right that langy can supply stainless pipe bends? I need some 2.5" for an exhaust I'm gonna make