Hobodog,
Um, I'm afraid to tell you its not nearly that much of an advantage over the 20s.
The .303 has a RoF of 20 r/sec.
The .50 cal has a RoF of about 12.5 r/sec, a muzzle velocity of 880 m/sec and 17,800 joules at the muzzle.
The 20mm Hispano MkII has a rate of fire of 10 r/sec, a muzzle velocity of 880 m/sec and 50,300 joules at the muzzle.