i dont think its short-changing at all, building in lighter weight costs money. the materials are expensive.
personally i would rather have a 1000kg AMG with 300bhp than a 2000kg AMG with 600bhp.
with both having the same amount of comfort and luxuries and quality etc.
same power to weight ratio, but the lighter version will be cheaper to run in fuel and in servicing and parts. and will handle much better and be in a different league of performance.
its the best way, aslong as top speed is not the priority.
but anyway AMG are going about it softly softly, they are not swaying the balance too much.