I read an article some 14 years ago....right after the MK5 came out. The article was one Tuner adding an Intake, Down-pipe, and then a tune. They did dyno(rolling road) pulls after each install and then following the tuning.
So, with it not yet tuned, they did some baseline pulls to see its all stock HP
Then the intake was installed and more dyno pulls. The results were a bit under baseline to 2 or 3 more HP. The average was zero added HP.
They then did the DP, and it was the same. A pull or two were under the baseline average and a couple up to 4 or 5 more HP. The average with both the intake and DP was 3 or 4 HP.
Finally they tuned it and finally saw the gains.
Adding an intake and DP to a non-tuned MK5 may add no more than 3 or 4 HP...some may see more due to the condition of theirs now, but the Tuner that did this was with a new MK5. They also figure Wheel HP on Dyno's here, so its BHP may be more like 4 or 5 BHP more than stock.