My guys at Buckhead Import Performance (very experienced on FI for many car makes) tell me that the non-M ECU is 100 times more complicated than the M ECU as it appears to them to have EVERY single routine and sub routine that the factory (or contractors) have ever dreamt up. Thus they make one change and the ripple effect thru-out the software is mindbending.
I am no expert but this is what they told me. Maybe some expert can post here a better explanation. BTW, it has been two+ years since my FI installation and they are still fine tuning mine on the dyno.