[DuSh]

M Sport exhaust = Stock exhaust in terms of sound - all 35i come stock with same quiet exhaust. M Sport only adds some details like different tips (trapezoid shape instead of stock round ones, and possibly different finish like chrome, not sure), but it's same exhaust sound wise - same muffler.

M performance exhaust - is a different muffler that produces more growl and a bit deeper sound. It has different construction inside muffler. Again, I'm not sure with what tips it may come with, could be same tips as in M sport exhaust, but sound will be different because of different muffler.

So basically tips are trapezoid or round, they may be chrome or not, but it's essentially just cosmetics. Only M Performance exhaust does really change muffler itself and thus sound

Hope this helps. Also, keep in mind that order will take time to arrive (you said your car is sitting in the shop waiting)

P.S. Just a rant. Dropping a $4600 for a M Sport conversion kit is very brave, but imho there are options to just imrove the look of your stock non M Sport look and add M Performance parts. I would have rather bought Aero kit for non M Sport and bought M Performance exhaust and M performance brake kit - basically for almost same money you'll get better sound, better brakes and upgraded looks (not M Sport, but still good), just my 2c. End of rant