Posting here for a friend who is having problems with his TT. He has an APR tune, Forge Splitter Valve, cone filter, and turbo back exhaust for mods. He said that he only ran 93 but he had to put in 91 recently and that's when the problem started. At the time, he put octane booster in. However, he has since used up all the 91 and is back on a full tank of 93 and the problem is still there.

His car starts misfiring/backfiring around 4000rpm WOT and loses power. It almost sounds like anti-lag. I attached videos (hopefully). Our first guesses were PCV or N75 valve, but we're both lost. A Vag-Com scan did not reveal anything.