Don't want to hijack the thread, but I have anecdotal woes of K04, and would like to add in and talk it through.
I have similar issues/quirks to this day. Although I'm APR and I want to blame the tune mostly, looking into Unitronic myself. I had a hard misfire "wall", like yours, replaced Cyl. 1's S3 injector recently and it fixed the up top RPM cut. However, now it misfires every once and a while at idle after it drops into normal RPM on cold starts, once it starts warming up. (EDIT* I do not have a RFD) After driving I come to an idle and it misfires more often after its fully up to temp. Same cylinder I just changed the injector on, tried plugs and swapping coils and still no luck, tried sourcing vacuum leaks, tried a new MAF and intake setup and nothing changes. Cylinder still shows signs on the plug of running lean, and fuel trims were unusually high.
VagCom is great, but logs have always looked normal, aside from those fuel trims, I need to check them again, but it hasn't told me anything aside from Cylinder 1 misfire as of replacing that injector and having a poor idle. Hell even AFR, vacuum, and RPM act steadily enough at idle.
For OP, I would do the injector and take a close look at any potential carbon build up in there. I need to do the same, but I really want to try Unitronic and replace some other maintenance items, and throw a catch can on just to see if that helps.
General questioning, does a poorly functioning rail sensor, or lower pressure relief valve, have any possibility of misfiring and leaning out one cylinder?
I would believe that would cause fuel starvation across the board. That would likely show up in VagCom logging as well. What else could lean out one cylinder? Even mechanically, valve seals, springs? Surely not at idle.
I want to say fueling is still my issue, but I understand an injector failing to produce at high RPM like mine originally. To be able to keep up under high load but not at idle seems illogical. I would guess to much air is getting in, but I don't have signs of a vacuum leak, and again would expect issues across multiple cylinders.
There's so much that can mess with the K04 tunes it's not even funny.
Sorry for rambling, but IMO more talk is better in the forums.
After rechecking fuel trims, idle adaptation sits richer, at -2.3%, while partial sits a lot leaner at 11.8%, what could cause such a wide swing between the two. My gauge always shows rich under boost, dips down into the 10s and 11s in AFR under 19-20 psi. I don't get it, it points to vacuum leaks but I can't source any. Resetting fuel trims makes it idle like garbage at first, then the live adaptation fluctuates between about -3% to positive 5%, looping over and over. Idle smooths out but I get a Cylinder 1 misfire and a puttering backfire every time it does, I just changed that injector, could it be a seal issue?