Well i dont know if I can be of much help... But I can tell you my story
But i had the same problem, and also had some mis-firing over 5000rpm in 2nd gear. I noticed it most after getting my car washed a couple of times - god knows why, I still dont know to this day.
SO - I re-gapped my plugs to 0.7mm, and cleaned up my coil packs (cleaned the outside of them as they had some crap on there) and this seemed to rectify the misfiring issue, but in the mornings I still get the spluttering when boost comes on.
I find that if i turn the car on, and drive away within 2-3 mins, it will splutter from about 0.5bar (7psi) onwards. But if i keep it under 0.5 bar for the first couple of changes, then all is sweet after that - i can run through to 1bar (14psi) with no spluttering/misfiring. It's almost like I have to warm the whole system up before being able to reach higher boost or something??? This does obviously make sense, but im lost as to the reason for spluttering
I have just accepted the fact that my car doesn't like being thrashed before it's warmed up properly, and thats that! It's definitely better for the engine to keep it under 3000rpm for the first few gear changes anyway, so I havent bothered going any further than this!
I hope this has given you some insight into what your issue may be. Let me know if you find any resolution for this