Wow, amazing story. Your project taking the car from "Disappointment but no surprises" to "I love it..it’s the best car I’ve ever had." is impressive and inspiring. I really liked the logic and choice in the name "MW Z4 M50i".
For perspective, my comments come from a guy with zero performance car acumen. Our Sdrive 35i is the only performance anything we have ever owned. We really like it. We don't drive it anywhere near its design capabilities as we just don't have enough motivation to find the right places to learn. However, I can relate to your thoughts about the run flats. I do not like the feel of the run flat tires but not sure how or if I will ever step out of the box and make such a change.
I read all of the words in the draft copy but given my performance car ignorance, I began skimming the text more as the technical nature of the information increased beyond my ability to understand.
I had to laugh at myself and my thoughts when I read your assessment of the stock 35i as being a "Disappointment". Some might read your story and decide the stock 35i is not worth buying. Besides the run flat tires, I really enjoy the car. Due to my ignorance, I am oblivious to the limitations of the performance of the vehicle.
Thank you for sharing your fascinating story with us. Your motivation and dedication to do the project and even write in depth about it is highly commendable.