I'd pressure test it. The supercharged cars aren't prone to UIM failures because the UIM is a metal supercharger whereas the non-charged motors are a plastic UIM. The LIM gaskets on our motors typically look horrible when we change them, yet they aren't a normal source of coolant loss. A couple years back there were a few show people here that liked to run around ranting and carrying on a great show. In the end, it'* was just a show. 99.99% of bad LIM gaskets look bad and don't create any issues, don't put coolant in the oil and don't cause a breakdown.
Being that you smell coolant, the chances of it being an internal leak are extremely low unless you are sniffing the tail pipe. I would start by looking around the engine bay at the throttle body, thermostat hosing, hose connections, radiator end caps, coolant elbows, water pump and heater hose fittings. All these areas contain coolant with a seal or something that can leak.
00 Regal GS: All kinds of cool stuff.