Sounds like there could an issue with your C-Max if it is roaring loudly enough to cause you to back off and irritate people behind you. Possibly the active noise cancellation is faulty. Not sure why the acceleration would be significantly different, maybe it's due to a difference in model year? The 2016 model year did add an ECO button which significantly dampens the acceleration, but the 2015 model doesn't seem to have a comparable feature. The C-Max is less aerodynamic than the Fusion. C-Max has a lower EPA MPG rating, so the slight difference in fuel economy seems normal.