Big shout out to
@wheela for pointing me towards the overboost (learned something new) & related wastegate action...
I decided to buy a new Pierburg (OEM brand) wastegate pressure converter and the car feels like it has a new engine fitted. Now runs totally smoothly and actually sounds very different - has a lovely smooth warble to the the tone of the engine now which it didn't have so much before.
This pesky part had me throwing money at the car and scratching my head for 6months.
Here's a video of how to install the part for anyone experiencing similar symptoms:
Took 30 mins to swap the part. All that is required is removing the intake pipe in order to get access. very easy fix from the top of the engine, albeit you need to concentrate on not dropping the lower bolt when removing it from beneath the sensor.
Here is an image of the part:
X3 different 'specialist' garages either failed to diagnose any problem at all or pointed me towards something else, unfortunately never to this simple part. I showed a BMW specialist the MHD log which was like trying to ask a dog to read Shakespeare. No clue at all. It just shows how valuable the collective knowledge of enthusiasts in this online community can be. Thanks again and happy driving!
A few additional notes about my personal experience with my N55:
If you feel a misfire, check plugs and coils first, and investigate a vacuum leak.
If your idle is rough, it is
most likely a vacuum leak. But be aware that a cheap smoke machine that has a weak 'built in' compressor rather than a separate airline feed may not detect a small leak that could be causing a symptom with your engine. This led me to think the leak was fixed when it wasn't fixed completely: the leak was only fixed to the tolerance of the smoke machine I was using.
The connection at the base of the air duct, where pipe 1 meets pipe 14, is very prone to leaking and on the e90 does not have a jubilee clip (both connecting hoses are hard plastic) as shown below. This I feel is a poor design.
Don't assume your car, if bought used, has a standard tune on it... mine didn't and was likely related to the poor running symptoms.
The MHD stage 0 is the same (I verified this with MHD) as a BMW garage will flash to the car.