What is the point of including an antimatter warhead on your missiles if they have such absurd performance? 

I mean really, 20 percent the speed of light? That's 59,958,491 meters-per-second, or roughly [i]134 million miles-per-hour[/i]. Any missile traveling with that degree of tempo is going to utterly obliterate any starship that it impacts regardless of the ship's armor's composition, the armor's thickness or  whatever hoodoo magic power field that protects it.