Since early 2022, Supersprint has developed a California emission compliant exhaust for the BMW G80 M3 / G82 M4, designed to fit both the RWD and the AWD versions.
https://bit.ly/3PjZlsG
It bolts right on to the secondary catalytics, requiring no modification.
This Supersprint system is a Plug & Play application, with the stock wiring harness for the flap valves being retained; no less important, it does not trigger any CEL error on the dashboard.
The rasp-free sound is night and day better than stock under all driving conditions, ensuring no droning in the low rpm range, at cruising speeds.
Also, it flows better than the Factory exhaust, both in the closed and in the open flap modes.
Here is a brief video of how it sounds.