I've seen so many theories of who it would be. I think they proved that their height changed from series to series, so I agree that the Stig does vary. But we all remember the famous interview, when it was the only time the Stig has actually spoken to someone on camera, and sounded remarkably like Damon Hill
As for special occasions (like the F1 driving scenes) I'm sure they hand the white suit over to a team-approved test driver, I don't think too many F1 teams would be keen to see a driver other then their own (despite possibly being a former world F1 champ) drive their cars.
But, such is the appeal of Top Gear. Have a person no-one (publicly) knows drive hard and fast, and watch the ratings climb. Matched very well with Clarkson's Tame Racing Driver introductions
