My understanding is that the ECU on GDI-engined +4s (like mine from 2017) is security/password protected from modification other than by the factory - I've had the "enhancement flash" carried out on mine (sent ECU to the factory), so did some reading around about it all.
When I had mine done (well over a year ago), the service manager at MMC told me that ECU-only reflashes would soon become impossible, as they would no longer have a suitable car there on which they temporarily install the ECU, to carry out the work.
Maybe over in the US you can get hold of a replacement ECU and then have a tuning provider tweak it to suit?
