I just finished working on an Ultramarines Tyrannic War Veteran at rank 5 (our game is about to reach that level after the next game session, and was planning on trying something new), and came across the Storm of Iron talent and wondered when to apply the x2 modifier to Magnitude damage, because it seems a little over the top when armed with a Flamer with a few other talents.
By default, the Flamer does 5 + 1d5 magnitude damage to a Horde.
Cleanse and Purify increases the magnitude damage by 1d5.
Tyrannic War Stratagem (Slaughter the Swarm) does extra magnitude damage based on Int or Per bonus (in my character's case, 4).
And then against Tyranid Hordes, due to Tyrannic War Stratagem (Cleansing Flame) I get to roll the 1d5 for the Flamer (not Cleanse and Purify) twice and pick the highest, giving me a chance to roll higher.
If all this applies before doubling the magnitude loss due to Storm of Iron, in 1 shot I would be doing a minimum of 22 (5 + 4 + 1 + 1 = 11 x 2) and a maximum of 48 (5 + 4 + 5 + 5 = 19 x 2) damage to hordes.
Compare this to a Metal Storm Heavy Bolter using the updated rules (RoF -/-/6), that can attain a maximum of 26 (6 x 2 + 1 for Explosive, and then doubled), a weapon that also needs to roll to hit first, and it seems a bit over the top.