Today I got the new gearbox control unit (black box) installed.
Hardware version is still the same (03/01) but the software version has changed from 24/02 to 44/02.
I can't say much about improvements right now because the new control unit has to "learn" after a fresh install but the problem I actually made a complaint about (shifting into 4th gear at around 50 kph and not downshifting if throttle is pressed by around 1/4 way) still persists. However, it seems that throttle response is much better (interesting!) and the downshifting works more spontaneous.
I'll give a more detailled report as soon as I get to drive my car for a few more days.
Right now I'm happier but not really happy.

The COMAND DVD-based Nav system got another OS Update today which really was worth it. Almost all bugs disappeared and I'm really happy about it.
Update took a few minutes, my mechanics at my dealer really rock because they came to me to offer me the update, I didn't have a clue that there is another one (just had an update a week ago).
Gearbox control unit install took a few minutes, they did it immediately. They had to initialize the control unit with the DAS system and with that occasion, they possibly found the reason for the problem I mentioned at 281 kph when the engine seems to have a short "hickup", dropping the rev by around 400-500 rpm.
The DAS system shows a faulty injection valve at cylindre 7 which actually was quite a surprise for us all.
Unfortunately this means that my car has to go back to the dealer to check on it further.
With that occasion, the battery control unit might be exchanged too.

Welcome to the 21st century, where car drivers play beta testers for manufacturers. At least I hope that the SBC software (brake system!) is bug free...

BTW: whoever makes complaints about Microsoft, my XP Pro installation runs smoothly without any problems. I wish I could say the same about my E55.

I'm lucky the mechanics (yes, around 4-5 are "playing" around with my car, I feel like a VIP ) at my dealer do a great job and show a lot of passion to get the problems fixed.
Seems that they see it as a personal goal to get ALL of my problems fixed. I like this attitude very very much.