... most drag queens--Hoffman, Weaving, Pearce, Stamp, Snipes, Swayze, Leguizamo--aren't really THAT believable. Would you say that women impersonating men, such as Julie Andrews and Hilary Swank, are more believable?
God is subtle, but He is not malicious. (Albert Einstein)
First, we should agree on a definition of "drag queen". The Wikipedia definition of it is "A drag queen is a man who dresses, and often acts, with exaggerated femininity." So, drag queens are the ones with all the glitter, feathers, big hair etc. Transvestites and transsexuals are more toned down. The latter do it because they identify with the other sex, so they don't really do it for the show but more for themselves. Thus, Jaye Davidson in The Crying Game was believable mainly because he was made to look like a regular girl. Lemmon and Curtis were made to look like this for the laughs. There wouldn't be a point to make them look very natural and real.
RuPaul is believable as a drag queen, because he's a natural performer. But he would not be believable as a woman, because not many women behave like that.