I love the little lock-on circle that pops up over the missiles and I love the minigun, it's a boat load of fun to use to shred ships up and addresses the minor niche of countering cinder block armor designs.
PD's themselves are significantly weaker now as i'm sure was intended and a lot of people have stated, looking at the length of this thread now. They are basically not worth the investment at this point. If we are going further down the rabbit hole with Flak ammunition being a thing, at this point just remove the PD part entirely if this is it's final iteration, and give flak ammo over to all the cannon parts.
The minigun and Large cannon as it stands are too valuable DPS to lose for anti-missile defense and they outstrip any usefulness the standard cannon can bring to the party. By removing the PD part and giving its current role over to cannon parts, the standard cannon can stay relevant to the game past sometimes being used in small ship designs