LoL, scuze man, I didn't mean to come off rude. I agree It is a rather interesting thread to bring up, just very hard to answer / add to accurately.
When i referred to OP, i meant to imply that 2.5" OD pipe is prolly cool on a maxed out TD05, but maybe not on a ie: T04Z@2bar, hence at a minimum, the power output of the car is a big input factor. Like most things, I woulda thought its about 'using the smallest pipe you can get away with' VS 'accepted flow restriction for vehicle purpose"
The issue is that rarely could i ever imagine someone taking the time to dyno and record pressure in various places in the pipe work, then replace ONLY the pipework and test for a delta. Most of the ppl i know, only change pipe size to match another mod being done at the same time, making "apples vs apples" hard. Reminds me of the thread a few months back "How much do large IC's effect lag / response" ... was lots of "heresay"