Hi guys I know this is an old thread but I think I'm having similar problem. The car has no eml light but no fault codes but seem to tail off boost around 4500rpm.
It's a 2007 S3. I've check the diverter valve and it's revision C but doesn't seem to have any splits in it.
Got it dynod at 180 from the standard 265. I've been to 5 different garages including audi and no one seems to know. Atm it's only got a cat back miltek exhaust. I did had a VWR Induction kit, forge pcv delete one and a Forge blow off valve adapter but audi suggested to put the car back to standard as they were certain the mods was the problem. I took them off and it made no difference the Maf g/s was 120 I got a new Maf sensor and ended up with the same reading.
Any help is greatly appreciated. Thanks