I just installed an MPE. Initially I was underwhelmed from inside the cabin, windows down. Too quiet. From outside, I think it sounds great - nice loudness, great sound character that's not stock yet strangely not aftermarket either. It really hits a sweet spot. The sound between 3.5K and 4.0K is really awesome! WOT is even more fun now. The engine sound is slightly amplified, as with an freer flowing exhaust, so the slight engine drone ~1.9K (present even with stock exhaust) is slightly louder but not at all an issue. Overall I'm very pleased with MPE.
I can easily see how you could go down the path of finding something that sounds good inside the cabin which effectively sounds obnoxious from outside the car and drones like hell inside. It's really weird but I'm almost bummed that I didn't take that path... going straight to MPE was almost too responsible, like skipping my teenage & college years yet ultimately the right thing for me. Even weirder is that I now understand/appreciate auto makers who are piping exhaust sound into the cabin via speakers (so long as it's the actual sound, not some pre-recorded crap). That may very well be my next "exhaust" mod... bizarre.