Non mandrel bent exhaust?

There is really no reason that I would have to go with a non mandrel bent set up other than it can be done locally by a regular shop and a little cheaper since I don't want to get the mufflers with the pre made kits. I just wondered if 3" crush pipes flowed close to 2 1/2" mandrel. It sounds like the strong concensus is just to go 2 1/2 mandrel from Summit etc.