Also, in "The Way of the Warrior," a task force of nine starships seemed to pose a serious threat to several dozen Klingon warships.
I don't think the military threat of the ships was a factor as such. Rather, these ships would be the official entry of Starfleet into the fray, transforming the whole situation politically.
DS9 was in Gowron's propaganda harboring Changelings and no doubt itself already co-opted by the Dominion. It's status was vague to begin with: formally, the Klingons were dealing with Bajorans there, and had nothing to fear from them in terms of retribution. Starfleet was not yet involved, not according to Gowron. Indeed, if he could kill Sisko, he could say that Starfleet had never betrayed the Empire by leaking secrets to Cardassia - only a now-dead Dominion lackey had done so. And the beneficial alliance could continue.
However, fresh ships coming from Federation space could not
be claimed to be Dominion assets, not without claiming that the UFP had already been corrupted by the Dominion. And that claim would be much more difficult to support than the one that the secretive Cardassian Union had become Dominion property.
Six ships would simply be too many to be made to just disappear, even if far too few to actually offer resistance to the Klingon forces. And for this reason Gowron could not fight them. He would very much have preferred to have DS9 destroyed before such a political intervention, and so would "Martok", but alas, no such luck.
As for a Klingon-Federation war taken to its conclusion, I'd think Klingons would actually be disinterested in winning. I mean, they love local victories, the total humiliation of the enemy, glorious looting, and so forth. But they have never shown a tendency towards the total annihilation of the enemy (save for Tribbles). For a warrior culture, total war is probably unthinkable...