Your damage calculation is wrong, the damage buff is way bigger. Just did a very quick test at the infantry test dummy in the keep. (Only relevant traits with my current setup for testing it on that dummy are 10% Power vs. Chaos and the 7% Power talent, I didn’t use Barrage for the test.)
Without the talent, the Drakegun does 750 damage per tick (and 350 per DoT tick). With the talent and on almost no overcharge the Drakegun only does 150 per tick (and still 350 per DoT tick), but that increases fast, and already on a little less than 1/3 of my overcharge bar I do 750 per tick again. On red overcharge, that’s increased to 1550 damage per tick. Which is indeed over double the damage per tick the Drakegun does without the talent. (The DoT remains 350 per tick, as you said.) Key point is that the tick rate when flaming seems to be the same as without the talent.
This damage increase allows you to kill more in a shorter burst. With a little tactical venting, you can contribute to monster damage suprisingly well sometimes. It allows you to roast a Hookrat, Leech, or Assassin nice and quickly, without needing a lot of time to charge the weapon before doing so. The limitation in overcharge room because you need to keep the Drakegun at higher overcharge is compensated a little bit by the need to charge less long, but the gain in charge time when you’re in a tight spot and need a quick burst of damage is the main benefit. It’s a net positive in my book, and then I haven’t even mentioned the no melee slowdown on overcharge, which is a godsend. Another aspect is that when you have your gun on about half overcharge, the leftclick burst starts being pretty powerful damage wise instead of only doing stagger, and it allows you to cook a Flamerat / Gunrat or even SV with two quick bursts, which can often save the situation as well.
The downside of the handling slowdown seems a fair price to me, and after a while I’ve gotten used to it and don’t blow up as much anymore. If you only use the Drakegun to cook hordes, it might well be more efficient to leave the talent and pick Blood of Grimnir, especially because that benefits your melee as well. But if you want a little more versatility against specials and monsters, the talent is actually a reasonable choice.
The only thing I’d like to see changed is to have its weapon swap after leftclick burst not be affected by the slowdown, because it causes some input buffering wonkyness. Maybe make the delay before the shot instead of after it instead?
This all is only about the Drakegun, not the Drakefire Pistols. On those your assessment of the damage increase on left click shots being minimal because of reduced shooting speed is completely correct. However, what that talent does for the Pistols is that is makes the shotgun blast do respectable damage, which makes it good for killing hordes in close combat. The talent basically turns the pistols into another weapon, where the left clich becomes its secondary function and the blasts its orimary function, instead of the other way around.
For the Pistols, that tradeoff seems not worth it to me, because if you want up close hordekilling potential out of your ranged weapon, the Drakegun is the superior option anyways. To me that seems more of a problem with the Pistols, rather than the talent, however.
The pistols could do with another buff, and in older posts I suggested to make the left click fireballs do really high stagger. That way they’re still capable of fighting armored specials, without making them too overpowrred because the killing time is still really low, and you can use them to quickswap and do a quick stagger shot where needed, which is an added functionality in close combat that fits IB’s role well.