I've read that you must be in quiet mode before turning off the car so that the exhaust valves are closed before you start the car (vs. closing a few seconds after start up in the previous mode). It might be worth trying if you're not already doing that.I'm just going to add a personal opinion here. Maybe there is something wrong with my mach 1? But I dont think so, I think it's just a natural beast. And I'm sure more people who have already done an after market exhaust mod can really chime in on this.
But my opinion: I wouldnt do an after market exhaust(already in my current living conditions and work schedule) just because of my neighbors. While I was waiting for my mach 1 to arrive, I went to a dealership, and I was able to sit in a regular mustang with a PP1 that they had delivered in the 1st quarter of 2021. My only goal of sitting in that mustang was to hear the active exhaust. I was literally SOOOOO AMAZED by how quiet the active exhaust was, just mind blowing. But my mach 1 doesnt even come close to how quiet the regular gt was. And I would be scared to make quiet mode anymore louder than it already is with an aftermarket exhaust. A few people have said quiet mode stayed relatively the same, but it already has enough of a small bark still to were I do wish it was a tad bit quieter(only in quiet mode, I would love it to roar a little louder in the other modes), but tbh, I cant control it beyond that, so I'm sorry neighbors. but I do live in a duplex, but it's not a side by side, it's an upper and a lower, and the lower is built into the ground, and my neighbors living room window is literally 2 feet away from my exhaust pipe, which kinda sucks. But If you own a full on house and dont have neighbors within 10 feet of your tailpipe, I'd definitely add an after market exhaust, but it has enough chirp in sport mode to where its unneeded for me at the moment.
If anyone wants to chime in with extra feedback on how loud their quiet mode is with their aftermarket exhaust, I'm all ears. I dont know if maybe mine is a little louder than intended, but its definitely louder than a GT in quiet mode. Like obviously noticable to me.
Congratulations on getting your car! Looks great!