Many thanks for both your valuable input, much appreciated this kind of quick support!
Pity that just this key sensor isn't released by aforesaid software tools, seems to me part of the basic data you want to read anytime. Well, fyi the car has been remapped twice, first one I had it reversed despite it went like a bullet (severe knocking in 4th at full throttle and increased consumption). Second one still in because I truly loved it the way the characteristic was improved without any downsides. Like it behaved as a petrol engine flying shifting up gears. Second remap was done by another professional company btw.
Only lately I got a Lambda error thrown up which I could firstly resolve by blowing out dry soot. But after about 10k it came back at which point I renewed the Lambda sensor. Strangely enough, the error code immediately returned and only after having it cleared several times in a row, it kept away for several thousand km's now. Readings are okay btw, as now.
The EGT sensor I had to renew previously at 80,000km (now 110k) due to going into limp mode right after almost every cold start. Replacing was the solution, but now again faulty would be unlikely to me. But still....perhaps?
Bad injectors could be, but there's nothing that indicates any such failure (correction values @2k rpm are between 0 and 1.5). No smoking or bad (cold) starts either.
MAF and BOOST readings are following eachother and rightly as desired at full throttle, however at cruising speeds in between, boost is ~0.2 bar too high compared to target. VNT lever is freely moving and vacuum is also checked. Even replacing a new control solenoid didn't make difference. I didn't open the housing yet as I don't believe the problem is that. Moreover because when it regenerates (between 300 - 500 km depending on driving profile), it there is no boost over atmospheric unless power is required.
Maybe I should consider investing in a scope as this is the one tool that's missing more and more diagnosing modern engine. For the moment I plan to go back to the tuning company and ask for their diagnose (as the nearby local Bosch Diesel Center isn't willing to look into non-standard vehicles).
I'll keep you posted once I found the root cause and solution to restore the performance.
Thanks again for any further input you may think of, cheers!