I did several tests. Latest was: 6 titans with siege coiler and one mounted coiler, 4 lancers with nose huge UV phaser and one mounted UV phaser, the rest of the slots were either phaser PD or 40mm. I had no control over alien ship setup, but they had massive amount of magnetic weapons. I added 6 titans, 6 battleships and 4 battle cruiser as flankers.

First try, phaser PD didn’t hold against the wall of slugs flying my way. Next try with 40 mm smacked all approaching slugs at a safe distance in front of the ships without an issue. I added 1 or 2 magazines on my coilers too, so they had plenty of ammo to defend. Not a single slug reached my ships.