If the cube is wiped in a single shot, there might not be enough borg left with enough data to adapt. They aren't adapting magically, they are able to analyze the shot and adjust their shields to block shots.
Ill bet, if they could sneak up on some cubes they could pop a few before the Borg really got wise.
I imagine the first cube would get some drones on the Death Star before being destroyed. It would take some time to assimilate the whole thing, so the Death Star might be able to take a few more cubes before being too far gone, but my guess is the first (or maybe second if the Death Star was lucky) cube would be the end of it.
Assuming the death star started firing as soon as they encountered each other it still takes about 14 seconds to fire. You can get a lot of data in that time.
You could also beam over a lot ( though you really only need one) of Borg in that time.
It also depends on the parameters of the shot. DS2 was shooting at partial charge to take out ships so there's less recharge time between shots. If the DS commander did the same to the Borg, they might blow a hole in the Cube without destroying it, and the Borg don't give a fuck.