The only "loss" is the option of a manual transmission -- which Buick is not known for.
Ergo if one is a fan of Buicks, a tossable shift-your-own hot trolley is arguably not what one is holding out for.
EDIT: After further reading, the Regal GS AWD is a Haldex-based system, not a rally-spec Torsen system. There is however a Quaife LSD available for the six-speed FWD car...