The most likely thing to cause an issue would be a dodgy boost/vacuum/PCV system with small leaks that you may not be able to see.
You can certainly do a visual inspection of all the pipework but failing that R-Tech have a smoke tester which will reveal how good (or bad) your boost system is.
The N249 by-pass is a worthwhile mod and I'm sure Nick can do this for you as well. Although doing this yourself will free up R-Tech's time.
If you have Vag-Com or can find somebody with it, give your car a scan to see if there are any fault codes and take it from there.
Fill it up with V-Power as well
