Something to be aware of with the SB12-Plus, great little sub though it may be, compromises are always made to get it smaller. In smaller roomer where size is an issue they can work out great. But I would really go with the PB12-NSD or the PCi 20-39 instead though if size isn't a deal breaker (which it doesn't seem to be if you're considering the PB12-Plus) for some real slam/impact especially in the low end above moderate volumes. The PCi may be a better fit in your room though as it has a smaller footprint, lighter, and (because it's lighter) easier to move. Which is basically a roundabout way of saying the PB12-NSD or the PCi 20-39 are both better performers than the SB12-Plus, there only downside being that they're larger subs.
A PB12-Plus in that size of room would likely be overkill for a lot of people, but if you like to turn it up loud or very loud then yes, you may benefit from the added cost. But I honestly don't think you would need it.