I have found some info on the net, saying that for some cars (S3, cupra, Ed30) the valve springs need to be replaced (FOC under warranty). I have some text and the TSB (Tech. Service Bulletin) number but I can't find the official document anywhere. Of course the dealer couldn't find it...
Fixing the valve floating and misfiring issues for free sounds great to me
TSB 2019062/1 Symptoms are misfiring between 6000-6500 RPM's under heavy load accompanied by power surging.
Technische Produktinformation
Vorgangs-Nr.: 2018868/3
Verbrennungsaussetzer bei hoher Drehzahl - 2.0 TFSI im Audi TTS und S3
Freigabedatum: 19.02.2009
P0303 - Zyl.3 Verbrennungsaussetzer erkannt (zu 80% werden auf diesen Zylinder die Aussetzer eingetragen)
Fixing the valve floating and misfiring issues for free sounds great to me