I found an article talking about the '67 Impalas use for Supernatural. Apparently creator and showrunner Eric Kripke originally envisioned the main characters getting around in some version of a 60's Mustang, but after talking with his neighbor who was a car enthusiast, the neighbor convinced him to consider the Impala due to its trunk space, as Kripke had explained to him the car would be sort of a home base, and needed to house a lot of weapons and equipment in their traveling fight against dark forces.
It says the character even states the engine specs in an early episode, which are not overly impressive, even for the time. However, it also says several of the actual cars used in filming production were souped up well beyond those stated specs, I presume in order to get better shots peeling out, appear more impressive in scenes, etc.
So like everything else on TV, sporty cars can't be trusted.