Ok - I have now had a chance to listen to several exhausts so I have some feedback for you and I made a decision for myself (just today so I haven't done the mod yet). While I will do my best to contrast the exhausts I have heard below, unfortunately it's clear that my opinion is very subjective so I would strongly recommend you listen to an exhaust you're considering in-person, as what's too loud for me might not be too loud for you, etc. As most others have said, listening to the videos online doesn't do any of the exhausts justice so you really have to listen to them in-person to be sure.
My Initial Objective: Add more volume in the cabin at lower RPMs for more noise during lower-end city driving. I have concluded that this is not really possible without a VERY loud exhaust due to the hardtop roof + noise insulation. So unfortunately I just won't have as much noise in the cabin as my E46 M3 Convertible (due to the E46's soft top), as I'm not willing to have it super loud outside because I'm a low-key guy. And I am not looking to add any power to the car - it's all about sound for me
My Final Objective: Add as much volume in the cabin at lower RPMs as possible (for more noise during lower-end city driving) while keeping it not too loud to not be able to have a conversation on my bluetooth headset with the roof down. So my noise limit now is being able to have a conversation with the roof down, despite the exhaust noise. I am not looking to add any power to the car - it's all about sound for me
With my Final Objective in my mind, here's my final verdict in order of best fit 1st, given my goals:
1) OEM Exhaust Mod (Non-Perforated - 2 Pipes) - My 1st Choice and What I Plan To Do - While I think this mod is about 20-25% louder on the outside than I would like, I'm hoping I will still be able to have a conversation on my bluetooth headset with the roof down with normal driving. I don't think a conversation on my bluetooth headset will be possible with hard driving but I'm not usually talking while driving quickly!! For the purposes of having as much noise in the cabin as possible under lower-end city driving, I would rather have my mod be slightly too loud than not loud enough, as not loud enough would be frustrating. So this is my #1 choice and what I plan to do. The sound is honestly not that much louder with the roof + windows up (maybe 20-25% louder) but it's better than nothing and is as loud as I would want to go on the outside. It will probably add a little involvement to my normal daily driving, which is my bottom-line goal. While I originally figured I was going to want to do the non-perforated 4 pipes, this is PLENTY loud for someone like me and frankly it might be too loud for some people out there who aren't used to modifying their exhaust (like me - this will be my first time). The best way I can describe the sound level is it's not quite as loud as an M6 Convertible (previous gen - V10) with the roof up (maybe 80% as loud) but it's much louder than the M6 Convertible with the roof down. The sound itself is FANTASTIC - like everyone says. Like I have read in other threads, it's a combination of bassy V8 + more exotic. It's not quite as perfectly exotic through the entire rev range as a car that is designed with sound in mind (like a Ferrari, for example) but it's pretty damn good. And it's VERY hard to beat the price (probably around $300!). It's worth noting that I was willing to spend up to $3,500 for whatever my first choice was, so the cost really wasn't a factor for me - just a nice bonus that it happens to be my 1st choice. As others have said, I would HIGHLY recommend that you listen to one, as it might be hard to beat. BTW, I disagree with some people who say that the car should have come stock like this. I think the car should have come stock with this type of sound but probably quieter than the mod sound level... I consider this a mid-range sound level for an exhaust mod in general (ie. louder than the Akra Slip-On Exhaust but not as loud as the ACM OEM Exhaust Mod), which means that when you hear it drive by you know its either modified or a VERY loud stock car from the manufacturer (ie. Jaguar XKR-S).
2) Akra Slip-On Exhaust - I liked the sound and it probably added about 10% to the cabin noise with the roof + windows up. If the OEM Exhaust Mod didn't exist then I would probably listen to other cat-back solutions but I suspect it would be high up on my list. The bottom line is that it just wasn't loud enough for me with the roof + windows up compared to the OEM Exhaust Mod. It's probably a great fit for someone who wants a better sound and a very slightly higher sound level. It hardly sounded like a modified exhaust to me in terms of sound level (not type of sound) when listening to it drive away.
3 Tie) ACM OEM Exhaust Mod - WAY too loud for me with the windows down so it would probably be WAY too loud with my roof down. Bottom line is that it wasn't a consideration for me because I can't see how I could have a conversation with someone on my bluetooth headset with my roof down. It's clearly on the louder end of the exhaust spectrum - probably more of a "Race" than a "Sport" exhaust in terms of sound level, at least over 3k RPMs. All of that being said, it's sufficiently quiet on the low-end to the point that if you drive the car very softly with the windows up then you won't hear much of a difference. But all other driving is too loud for me. It's actually almost the opposite of what I am looking for - quiet on the low end and VERY loud on the high end.
3 Tie) M Performance Exhaust - Nice sound but it doesn't add much (if any) sound inside the cabin at anything under 3k RPMs. Very, very similar to stock sound levels below 3k RPMs. Above 3k RPMs it definitely has a nice sound that is different than the stock exhaust, as most people report. But it just doesn't mesh well with my goals.
I left out 1 more exhaust I heard - the Akra Evo - only because it's not the cat-back mod I'm looking for and is far beyond my budget, so it really doesn't belong on my list.
Anyhow, I hope that helps. I'm going to get the OEM Exhaust Mod (non-perforated) after my initial 1,200 mile service in 1-2 months and I will prey that it's not too loud for me to have a conversation on my bluetooth headset with my roof down. I pretty much won't know until I get it so I'm willing to take the risk, as I can always put the roof up for an important conversation if need be. I'd rather trade off having to put the roof up for that need vs. not doing the mod, as it sounds too good and frankly I'm just too unhappy with the car's sound stock (it's too quiet, especially with the roof + windows up!).
I will report back after I do the mod to let you know my opinion about having a conversation on my bluetooth headset with my roof down but I anticipate it will take a good 1-2 months. I think my countdown to the 1,200 mile service is something like 350 miles but it will probably take me 1-2 months to drive that distance!
Hope this helps,