The car cannot tell if Technic's harness is present. I suspect his computer is looking for a coding harness. What is your BMW specialist using for recoding? Is he the authorized BMW dealer? Is he using an AutoLogic?
Once you recode to HiFi you cannot run the speakers without an amp. The voltage drops from 9V peak to 5V and your speakers will be faint and almost impossible to hear.
I categorically disagree with your statement that recoding is needed. We do these every day without recoding if done properly. I don't see how you're going to get better results with HiFi than with Stereo with an MS-8 myself.