I have driven several cars @ 20psi (both stock turbos and 2530's). Stock feels awesome @ 20psi, but I wouldn't do it much.... You will damage something eventually. And yes Mike has turned a 12.1 or something on stock turbos w/ full slicks.....so it is possible, but takes a hella good driver.
And yes, 20 psi on stock feels awesome (w/ very little lag), but try it on 2530's.... waaaaaaaaaaay better. 25 psi is truely fun ;)
anyway, just be careful, 20psi is pushing it on stock turbos.