Why would the mirror universe Borg be any different then the regular universe Borg? The main reason the mirror universe we saw was so different is because earth's history took a vastly different turn, and since humans had such an effect on the course of Alpha Quadrant history, the mirror universe history was dramatically changed too. Tucked away in the Delta Quadrant I imagine history for the Borg went on much as it did in our reality.
I agree.
I the DS9 and ENT MU eps, it seems as though the only culture that is different is Earth. The Vulcans are still passive, the Klingons and Romulans are still aggressive, etc. Only Humans are different. There's no reason to think the Borg would be any different.
Calling the MU the MU is really a misnomer. At best it's only a Parallel Universe that is somehow closely linked with ours, thus explaining the ease of crossover.