After about half a month of grinding for the requirements, and pausing progress on the Pistols directive, which in turn paused all of my other in-game goals, I finally got the Firestorm implant.
Here's what Firestorm does:
Tier 1: Killing an enemy increases fire rate by 8% for 1.50s.
Tier 2: Killing an enemy increases fire rate by 8% for 1.75s.
Tier 3: Killing an enemy increases fire rate by 8% for 2.25s.
Tier 4: Killing an enemy increases fire rate by 8% for 3.00s.
Tier 5: Killing an enemy increases fire rate by 8% for 3.00s, or 5.00s if a headshot.
On all tiers, recoil and cone of fire (CoF) bloom suffer significantly while the effect is active.
Maximizing the effect
To take advantage of the increased rate of fire (RoF) within the 3 second window after a kill, one would have to be able to chain kills one after the other in rapid succession. In achieve this, there are three limiting factors one must know:
Reload speed. The biggest enemy of this implant. That's because the minimum average long reload duration of any primary weapon is about 3 seconds. That's the same duration as the effect. This means that if you reload, you're guaranteed to lose the effect.
Magazine size. Aside from finding ways to reload faster, another way to avoid reloads is to have a larger magazine capacity. The more rounds you have, the more time you have between reloads, which means more time to rake in kills and reset the effect.
Accuracy. The debuff was introduced to counteract the powerful DPS buff the implant provides. This means weapon accuracy suffers when the effect is active, limiting your ability to just rake in kills. To maximize the effectiveness of the implant, this debuff must be counteracted.
The two playstyles
Now while trying out the implant, I've found two play styles that take advantage of the effect.
The first play style is the obvious one: A heavy assault with a heavily-compensated LMG. LMGs are good candidates because of their high magazine capacity and their various accuracy attachments that negate the accuracy debuff. That coupled with a conservative play style (taking cover, burst firing) allows good use of Firestorm despite the debuff.
The second playstyle I discovered by accident while auraxing the T4 AMP. It's using a pistol with Firestorm together with Gunslinger. This playstyle takes an already fast-reloading weapon and further reducing its reload duration. Additionally, pistols have very low CoF, bloom, and recoil, making them the perfect weapon to apply this implant to. Put this combination in the hands of a stalker infiltrator, and you're golden.
Other playstyles don't really maximize the use of the implant. For example, I tried this on my SMG infiltrator loadout. With an Armistice, I can take down 2-3 enemies before my next reload. But with a 30-round magazine and a 3-second reload duration, Firestorm is only in effect on the second and third kill. Same story for carbines and assault rifles, not a whole lot of benefit.
The main effect is reasonable, three seconds is long in any game and 8% increase is not noticeable really. I just wish the debuff was something else. It could have been made similar to Assimilate where one would need headshots to gain the effect. Or it could have been made similar to Deep Operative where the effect is removed once damage is taken.
Overall, it's a decent implant. But it could have been better.