Getting a sub that can fill your room and more isn't overkill. It's called 'having headroom'. The less you have to drive a sub to its limits, the more refined it will perform.
In a small room, floorspace may be the restricting feature. You should also look at the PC12-NSD as an option. This has a smaller footprint, but is 35" tall. Get the one that best fits your room. Either one will kick a$$ in your HT. In your smallish room, you might also consider the SB12-NSD. You should get significant room gain down low that would give the little sealed sub quite a performance boost where it could actually outperform the big ported versions over a wider FR.