I compare the MS3 exhaust note to the exhaust on my recent MSP. On the MSP I ran a 3" DP-back custom exhaust with a single hi-flow cat and a Magnaflow deep-sounding muffler with 4" tip. From the outside, the stock MS3 exhaust sounds the same, with the same volume as the exhaust on the MSP. From the inside, however, the MS3 exhaust is quite a bit quieter as the MSP exhaust was unbearably loud in the backseat for any period of time, and a bit much to handle in the frontseat on extended highway trips.
To answer your question, I think that it would be as loud as hell since the exhaust is already noticeably louder than your average exhaust. Whether or not it would be bearable from within the car is hard to say as I think that the MS3 is much better sound-insulated than was the MSP.
R