i would guess because the factory dme doesn't have a way to measure emap. (exhaust manifold absolute pressure). for those running standalone you can add sensors and catalog this .
best comparison for exhaust is from stock to modded exhaust. don't know of any oem stock cars running standalone

 

My post was meant to be sarcastic. The one of the easiest ways of measuring exhaust performance is by testing back pressure and nobody(that i see) does it and posts results. Seems europipe is the only one.

Ps- it is nothing to add a few 1/8" bungs and a pressure sensors. Then just data log with anything that can log a 0-5v signal

 

Exactly. Whatever the part is, it is crazy that people are buying this stuff with minimal top no data. No Porsche engineer worth his **** retentive degree would ever buy parts without full specs relevant to that part. For intercoolers, I'd like a 0 to V-max run before and after with car specs outside of the single part being tested, test atmospheric conditions, and corrected to standard atmosphere, speed versus time, accelerations, inertial dynamometer values for the run, before and after temperatures, and backpressure. For exhaust and turbos, the same. I'd also like the data for max throttle from 1000 rpm. Be careful, at low RPM, a car may never make the same torque in reality as it does in a slow dyno run because it will accelerate past those RPMs before the turbos spool up. I'd rather have good flat road inertial (accelerometer in the car) runs than numbers from a car strapped down to a dyno.

 

For turbo's - temps, backpressure aand a turbo speed sensor works well in determining the proper turbo wheel sizes. You will find those turbo maps pretty accurate when you have the proper data in hand.

The factory exhaust does have bungs installed. I am going to make my own exhaust soon but, it will not likely fit your "quiet" requirements. Although, I have been using the TT as daily driver lately and sometimes I would like something quiet too but, not really a concern right now.