If you had watched more of Voyager you would know the Borg have a whole transwarp infrastructure Galaxy wide, hundreds of thousands of ships and probably trillions or at least hundreds of billions of drones. Not to mention an industrial potential much greater than the dominion.
A full scale war would se the dominion probably winning many victories and the Borg suffering many casualties but they'd keep coming and coming and would adapt and sooner or later they'd assimilate a Vorta or Jem'hadar if not a changeling.
The Borg would win.
The Borg never sent their full might against the federation and Species 8472 had a biological resistance to the nanoprobes and were super advanced themselves.
I know the kind of numbers Voyager gave to the Borg, and this is exactly what I've said the entire time. My point is simply that the Changelings are potentially the most dangerous adversary the Borg could face, with the possible exception of 8472 (the biological effect of nanoprobes on Changelings is entirely up in the air, and it would be very logical to say they have a similar resistance; 8472 has the advantage of Fluidic Space as home turf which the Dominion doesn't have, but a single changeling, if immune to nanoprobes, could be as effective as dozens or hundreds of 8472 in sabotaging cubes; 8472 do seem to have better technology than the Dominion), and that the Dominion would easily handle any Borg attack on the scale of what the Borg have attempted against the Federation. Yes, of course, hundreds of thousands of cubes cannot be stopped by any known (normal) civilization, and I don't think anyone here is actually arguing that. But, at the same time, that's exactly why it was a terrible idea to make the Borg so expansive in the first place, and exactly why TNG and the other shows always featured limited numbers: because if you give the borg a massive numerical advantage on top of their massive technological advantage, then there is no story left to tell.