I'm not sure if you'd be able to get an erection with a 20 inch penis. All of the blood that would be leaving your body and going into your penis is liable to make you pass out.
About reaching 20 inches with PE: Right now, with the knowledge we have, the idea of gaining that much is far out there. Why?After a certain amount of gains, most guys reach a point where a) It's harder to gain and b) They don't want to gain anymore. Perhaps in the future when we know more about our bodies, about PE, and about supplements that help, we'll be able to gain that much. . . . But I'm not sure why anyone would want to!