For starters, I'm pretty sure we've actually seen Jem'Hadar drones ("Infinite Regress" I think?). And if the stardates of "First Contact" and "Rocks and Shoals" are as they appear to be*, then the Borg might've already assimilated the means to adapt to phased polaron beams.
*This goes waaaaaay back, but I remember Mike pointing out that the stardate cited in "FC" corresponded with one in DS9's early sixth season. If you go by that, vulnerability to the Dominion's shield penetrating weapons could be null and void. Of course, "FC" was in movie theatres much closer to the time of the "In Purgatory's Shadow"/"By Inferno's Light" fifth season two-parter, in which Sisko specifically mentioned that, between the "... war with the Klingons and the recent
Borg incursion, Starfleet's spread pretty thin."
Personally, I choose to believe the stardates got mixed up and the Borg did the "FC" thing in DS9's fifth season. That makes more sense to me than A., referring back to Wolf359 as a "recent Borg incursion" or B., a Borg ship attacking offscreen just a few months before "FC"
That really doesn't mesh with the context of "FC" in general ... for one, Picard and company seemed surprised that the Borg were back. But if the Borg just popped up less than a year before, well ... that rather spoils the girl with the giant tits popping out of the birthday cake trick, doesn't it?
End aside.
Apart from that, I don't know squat about Star Trek: Online, just that some footage I've seen makes it laughably unbalanced and unrealistic. I literally saw a small KBoP ripping apart several B-Type Warbirds in one engagement!
That's just fuckin' silly, ladies and gents.
However, I can say with great confidence if the Borg Collective decided to invade the Dominion -- and I mean, truly
invade, as in sending hundreds or more cubes -- how would the Dominion stand a chance in hell? Their bugships, which compromise the vast majority of their navy, are tactically inferior to scout/raider-type Klingon Bird-of-Preys. Their battlecruisers are perhaps on par with a GCS (maybe a little superior, maybe not; in "Call To Arms," Dukat's flagship only took a couple of torpedo hits, and the second not only defeated his cruiser's shields, it blew a hole straight through its port nacelle), and their battleships are "three times as strong" as a GCS. Individually, none are a match for a cube. A small fleet of them are no match for a cube. Ramming is cute and all but the attack ships aren't very big. Even a swarm of direct ramming maneuvers shouldn't do crippling damage to a ship that's 28 freakin' cubic kilometers ... assuming the Borg just sit there and let themselves be rammed, that their shields don't stop any of those attacks (y'know, like DS9's did when a Hideki slowly crashed, and subsequently vaporized, when it rammed OPS' shield), etc., etc.
Even without access to their transwarp network, the Borg have transwarp drives with limited range ("Dark Frontier"). Those drives are fast enough that they could show up all over Dominion territory while the Jem'Hadar scrambled to intercept them. And assuming it did come down to a large fleet battle, I simply can't see the Jem'Hadar winning without a 20:1 numerical advantage. Even that strikes me as generous. The cube at Wolf359 plowed its way through 40 starships, and it was the sole target. Imagine trying to coordinate an attack against cubes that occupy your entire field of vision.
To me, the Dominion is like a "grimdark" Federation. They have a massive fleet, nasty stormtroopers and do whatever needs to be done to enforce the Founders' will. At the same time, it's quite clear the Dominion doesn't field ships with vastly greater firepower, shields, speed and armor than any of their opponents in the War. Their biggest, baddest ship probably outgunned anything the Allies had (maybe even including superwank ship
Scimitar, which I remain convinced was a design Shinzon stole from Vorta engineers), but they were giant slugs and incredibly few in number. Shields? Like the Cardassians, I think the Dominion navy depended more on their ships' armor and overall integrity than anything.
There's nothing wrong with those things, mind y'all -- I just don't see how the Dominion would fare any better than the Federation, Klingons or Romulans vs. the Borg. Their shipbuilding capacity's undeniable but, as I said, just what kind of INVASION are we talking about? If the Borg are determined to conquer Dominion space, I've little doubt they have the numbers to do just that, and in steamrolling fashion.