I was thinking about the exhaust system including the catback. I do remember the obvious gains on the 3" DP/TPs, though.
For example: If someone has a full turbo-back 2.5" system, going to upgrade to 3" DPs and TPs, that would be great, but adding the expense and weight of a 3" catback at that level may not be as beneficial. That's what I meant earlier.