I have for sale a used RPI GT full exhaust from the downpipes back. Would like a local sale but I can ship it at the buyers expense. Let me know if you have any questions
Probably the closest exhaust I have found to make the time more like my E92 M3!
Site states it fits both the F80 M3 and F82 M4. This was on a 2015 F82 M4 for reference.
Located in Northern KY (Cincinnati) area
Sound clip in video below: