Double Volley is great for three reasons: +30% damage is better than +15% damage; enemies tend to die before the +15% damage dot expires, meaning you're getting less than the advertised +15% damage; and double volley is bugged so that it requires two lock breaks to prevent both missiles from landing, so you're almost always going to get 65% normal damage and often 130%.
You obviously want the ammo boost in t4 to go with it, and it's worth looking at the magazine and crew member options to extend your ammo limit even further. I personally learned to fly on a Flashfire with Kira Carsen, giving me 18 clusters, and so I don't use the ammo capacity extender.
It's also worth noting that in TDM, yellow powerups will restore some of your used ammo.
Another important thing to note: Double Volley does not affect the crew member capacity bonus. On a Flashfire, the base capacity is 20, with both capacity options taken, it increases to 30. But with Double Volley selected, you get 18 missiles, instead of the 15 you would expect.