Well, in that perspective, O.K. I re-read my post and never actually gave a failure curve. Around here, we only get seasonal use. In the first year, they were repaired twice. repair cycles only went up from there.
I agree that failure is from fabricating/design. Thats why I stated that a few slip joints would probably saved or prolonged their use. With that said, heat cycles DO harden and shrink the steel. If that isn't taken into account in the design process, they will fail no matter who welds them.
Also... I agree with Fred. Headers might make more power on an engine that is maxed, but in my case I never even got close to that point.