Since we have been making the long splits we have been measuring before and after grinding the inside welds and we have been adding to tack welds to the outside for extra strength. By tacking the outside we are still allowing for the pipe to expand and contract without cracking the outside weld.
Taken from his post on the subject in yesterday's thread here.
I'm sure he knows what he's talking about here and I don't think there are any reported cases of the new and improved downpipes failing.