Yeah I haven't heard great things on either the E46M or Z4M side regarding ARH 6-1. Fitment was apparently difficult on the Z4M, and I don't believe anybody has fully fleshed out a proper tune for it (compared to say an SSV1 which is well supported with proven gains).
Here's a thread with more detail:
https://www.zpost.com/forums/showthr...1397666&page=2
Regarding fitment. I believe in >90% of cases the headers are compatible between the platforms, however I'm nearly positive that's not the case with the Section 1's and there needs to be a Z4M-specific fitment.