There are actually no serious studies that show any clear superiority of the Dvorak layout. Those usually cited were basically conducted and crooked by Dvorak himself; he used all kinds of tricks to make the Dvorak typists have better results.

Studies done by independent parties (one in 1953, done by the Australian Post Office, another done in 1973 by Western Electric) found only insignificant (less than 5%) differences in the typing speed of people intensively trained (about 100 hours) to use the Dvorak layout, compared with their previous performance on QWERTY, and the differences went both ways.