Maybe, and this is just a shot in the dark, but maybe the reason to move the exhaust down is to clean up the air going to the rear plane wing. The wing that goes over the rear crash structure. It probably receives denser air and hence is more effective, when you don't have hot exhaust gases billowing towards it. By comparison, this wing is very plain on the Mclaren, but other teams have it much more sculpted. I'm talking about the wing where the bridgestone logo appears.
Look at other teams, the Red Bull for example has this lower wing mounted higher, and the rear of the car is probably sculpted to help feed air to it.