Don't confuse brass with the factory ammo. Just because fed GMM shoots well doesn't mean that the same load inside of someone else's brass won't shoot better. Of course you won't know til you start loading.
Figure this way, you want to get brass from store bought loads(which you are over paying for) and you're getting sub optimal brass. Or you could stop buying the factory loaded ammo, start reloading, buy the good Lapua brass, and in 6 months be ahead on money if you shoot enough for reloading to save you money in the first place.