My two pence worth.....
MMC want/need to improve their overall average emissions level, so whilst they could keep the big V8 (It is Euro 6) it is effectively unsaleable outside the UK, Germany and Sweden because it has CO2 levels over 250gm/km.
So they need something with about 300bhp and a CO2 level of something like the B48 which is a Turbo 4 that can produce 250bhp as fitted to the 730 and and in a car of the size and weight of an Aero would give CO2 levels about 130gm/km, making it attractive in many more markets.
Perhaps the related straight 6 would fit, this is the B58 that is used in the M240 and gives 335bhp and would still be less than 190gm/km....
Aero 4 and Aero 6 anyone?