That Mercedes-AMG wants this motor to sit low and far back in the AMG GT chassis. The dry sumpy lubrication system allow the motor to sit 55mm lower than if it used a conventional system. Additionally, as the M178 is smaller at 4.0 liters than the M157 5.5 liter V8 many were clamoring for this lower center of gravity combined with a lighter motor means a car with better weight distribution and of course less weight on the nose.
This is important as Mercedes is targeting the Porsche 911. By keeping weight balance toward the rear Mercedes will not allow the 911 to have a huge advantage thanks to its rear engine layout which pays dividends in traction off the line and braking performance. So Mercedes likely has a 49/51 front to rear weight balance going thanks to the dual clutch transmission transaxle setup that they are mimicking in the AMG GT from the SLS. The AMG GT will not suffer from the negatives of the 911 rear weight bias which usually means slower entry to an apex but quick exit speeds thanks to being able to get on the power earlier. It also means no problems with snap on oversteer. The AMG GT will be better balanced.
Turbo spool should actually be better than the M157 despite the displacement disadvantage. Why? The M178 offers a 10.5:1 compression ratio whereas the M157 offers a 10.0:1. It is not just the compression ratio but the turbocharger manifold design which will affect spool and the M178 features shorter routing of the exhaust gases with its top mounted turbos. Mercedes built a very efficient design here clearly taking throttle response and spool for the track into account.
This is going to be an exciting motor that will be with us for some time and a worthy successor to the great AMG engines that preceded it.