Ok so as a 318 owner i will chime in. The m42 does NOT have fart can sound properties at all. If you install a fart can it still has a distinctive sound, bad, but still different.
As for me, I still have my stock exhaust with a hallow muffler. Benefits: no drone, deep idle burble, nice deep sound without being loud and obnoxious. No attention from cops.
With the windows up, the car is still quiet as there is no resonance or drone. Windows down , a pleasant sound which comes out on hard acceleration but is almost not there when you want to blend in.
Loss of power on low revs. There is a distinctly lower powerband when not revved up. That said I also have a intake, but now that the exhaust is free flowing for the most part, i lost some get up and go till about 3k. Then suddenly the car wakes up with a vengence.
MY opinion, do it. It will give you a nice subtle tone without the ricer sound or feel