There have been a number of Stigs, all of them racing drivers of one sort or another. Apparently the BBC doesn't pay much so they tend to be from the ranks of the formulae somewhat below F1.
Mostly ben collins, ex team mate to takuma sato in british f3 and sometime sportscar driver. Was on top gear before racing a downhill mountain boarder in a bowler wildcat and also the car football episode. Good driver. Nice bloke!
just got back from seeing the show recorded, its a good one this week... however can't confirm who the stig is as i didn't see him... but in the past i think it has also been james kaye.
as for stig being quicker and not making it to f1..... that's motorsport i'm afraid.
For run of the mill road cars the stig is who you mentioned "BC" but when its a high profile car like enzo,lambo,etc its there test driver dressed as the stig...