We deffinately need to see that pic. If he's wearing a shirt the same color as the car he's probably a team member, especially if there are sponsors on it. It's entirely possible that this wasn't the regular Stig. I keep thinking about his helmet. They probably wouldn't let him use his regular helmet in this car but did they bring a white one just for him? And even if the guy is about the same size as the Stig, I think that's far from conclusive.
Just some ideas.
Also, if the guy seemed comfortable in the car then he probably isn't a guy who's just sitting in it the first time like ol' Stiggy would be. The preceding has been another thought.