Hey FunkySpoo,
Regarding your questions, while I don't have specific information about the Berg 2x15, in general players are finding that two of the BA12-3 easily replaces much larger rigs. One recent player moved away from a major brand 410/15 cab (all in one) and replaced it with the BA stack. His bandmates can't believe how much better they hear him.
This gets to the projection and dispersion characteristics of the Bass Array. As you mention in your post, sensitivity is rated at 1W/1M which is correct. What you get with the Bass Array design is how well it holds together at 15 to 20 feet in front -- as well as off-axis. This is what is being reported by so many users on the forum here. This is where this design really changes how players hear themselves in a live environment.
There are many reports from players that have normally used 15's that there perception has changed after using the Bass Array12-3.