I think they(GM, but really Ford and Chrysler too) should do more performance flagship large sedans, but that's because in my intuition I don't think the incremental cost of these things is that big. Most of the cost of engineering a new model/platform is in the design of the unibody and the styling, I think. Simply adding a larger engine and other performance upgrades actually doesn't cost very much, in the big scheme of things. They get to charge a price far higher than what it probably cost them to build them on top of what they've already spent to make the base model. It won't be *profitable* because they obviously don't sell in big volume but the marketing/brand image benefits are huge for something that really doesn't cost much.
Again that's just sort of my intuition on this, I don't actually know for a fact if that's how it works.