BRASELTON, Georgia — Three years ago, I watched here at Road Atlanta Raceway as a blood red, V-8-powered Cadillac DPi-V.R prototype streaked ahead of a snarling pack of IMSA race cars into Turn One on its way to a convincing victory at the Petit Le Mans 10-Hour endurance race. The Cadillac was the class of the field. This year I returned to Road Atlanta driving an XT5 SUV, the best-selling Caddy in the luxe maker’s lineup. There is an unmistakable similarity between the DPi-V.R and XT5. They share Caddy’s signature tear-drop headlights, vertical taillights and brand logo on the nose. And that’…