I think most engines running at 2500 rpm for 20 minutes straight likely also have the added benefit of cooler surrounding air whipping around the headers at 60+ miles per hour and also likely receive periodic breaks of lower rpm while decelerating and going down hill etc, and 800 rpm idling etc….
I doubt the headers in use in a car get as hot as ones just sitting on an engine stand while a fresh tight engine runs for the first time at extended periods (20 min +) at 2200-3000 rpm.
I think all the manufacturers recommend to not do it because they got tired of fielding calls from people who thought they knew it all, had it timed, jetted perfectly, and accounted for every possible little thing that would make their engine not run hot on the break in stand.
I think Most people, if they have the means (ie old headers, or manifolds) lying around use them on the run in stand or in the car for the first time, and then preserve there $1000 investment on the new polished ceramic coated headers in box in the corner of garage waiting to go on after all is broken in and sorted out.
This is kind of a crazy thing to argue about.
