The reason Fire is a waste is in the meta. You can only have ONE stack of Firestorm DoT damage on a target at a time. At best, extra shots just refresh the timer on the dot. You can spread the DoT to a lot of targets, but they’re not going to take more than 96 extra damage from it starting from the point you stop damaging them.
Ice bullets, or as I call him a Chilly Monty, do more for you over the long term. It’s Monty’s BULLETS that really lay in damage. My tankiest Monty for example, doesn’t use extra health or shields…I use Attack Speed and Monty’s Legendary. 
Because I’m shooting Ice bullets, I’m slowing targets, keeping them lined up for the minigun longer(more damage). Because Hailstorm can eventually penetrate targets, my ice slow, becomes an Ice CONE. damaging and slowing all enemies in a lane if I want. Because I’m firing faster with my AS items, and i’m cooling my minigun down faster with Hailstorm(remember, Level 4’s The Cooler, decreases heat per SHOT fired. More shots fired, faster heat drop off.), I’m able to shoot LONGER, and stay in the minigun’s hotzone of damage longer, while taking less damage and dealing more damage. 
Fiery Monty’s work best in PvE, where you won’t find too many targets that are really hurt by a slow. PvP, is ALL about the Chilly Montys 
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