Last thursday our school had a visit from a guy from Sunday digital, who is making BMW commercials. I showed him my work, and got some feedback. In my scene i had simply rendered a car using HDRI lighting with a matching backplate. He said that, what they typically do is add additional fake lighting to the scene, or rotating the HDRI and making multiple renders and then combine them in photoshop.
He also said, that when they make photography of cars outside, normally what they do is that they get to the location early in the morning and start shooting photos there, and then take shots throughout the day so that they have different lighting. Sometimes they also rotate the car a bit, to get lighting to hit other sides of the car, or maybe get a nice light gradient on the side etc. so they can mix that in photoshop.
They also some fake lighting like you said with flash- and spotlights.
I'm no expert, but it has seemed to work for me.
I've attached a photo that shows before and after of using 4 renders each with the same HDRI just different rotations. I think it has improved the lighting.