I drive from McLean (GW Parkway) to Potomac (Democracy Blvd exit from 270) via the Beltway numerous times during the week, both during morning rush and early afternoon. At 5-6 am, there is virtually no traffic, so it would probably be much faster than taking public transportation.
Coming back might be a different story, depending upon the time you are leaving. My experience on the stretch from 270 to the GW Parkway is that it's pretty smooth sailing until about 5 pm.
Keep in mind that the recent bridge-painting project at the entrance ramp onto 495 from the GW Parkway is having a major effect on traffic. But not at 5 am.