On top of that, we essentially have no reliable figures for Star Trek powerplant outputs, and the damage they seem to cause with their weapons is inconsistent. Photon torpedoes are measured in 'isotons,' but have been demonstrated to be able to vaporize cities in a single blast, indicating a yield of at least a few megatons. Quantum torpedoes being significantly more powerful, and transphasic torpedoes a few magnitudes above even that.
There's also the matter that according to the DS9 Technical Manual (based on production notes, and considered mostly canon), phaser rifles can cause major ground displacement (e.g. collapse entire cliff faces) if set to maximum power. Then there is the transporter rifle used in DS9 itself to shoot through walls.
IIRC, the Empire at the peak of its power manufactured only a few thousand Star Destroyers and less than a half-dozen Executors. Even if they went on the offensive against the Borg, it would still take them much longer to destroy those millions of cubes than it would for the Borg to adapt. If even a handful of cubes survived, it would be over for the Empire. And I doubt the Empire is clever enough to use Voyager-esque finesse to kill the Borg Queen. Even with the Force, there is only so much that can be done, as indicated by the fact that the Clone Wars were largely decided by combat between conventional troops. At the height of the Old Republic, the Jedi numbered only a few thousand, most of which were not terribly powerful, and despite outnumbering the CIS force-user corps several thousand to one, the might of the CIS' droid forces held the Republic back for years.
Also, correct me if I'm wrong, but to travel via transwarp don't the Borg need to use per-constructed Hubs?
To travel via transwarp conduit they did. But the hubs only maintained the network and served as a transfer point. Once the hub was built, any cube with a transwarp engine could access one of its nodes in any other system, opening a portal into the network. They could also travel via transwarp without the conduit, but this was slower. The conduit would allow them to reach the Alpha Quadrant within hours, whereas transwarp would still take a long time.






