Can justify the cost of 2 SB16's thought for the price and my room size, which is 11x16&1/2, and the fact that it's total overkill.
Define "overkill"? On most HT forums that word is considered heresy.
The major advantages derived from dual subwoofers come about if you have flexibility in placement and are comfortable tuning them. Without both, the value they can provide diminishes significantly; aesthetics or symmetry can't dictate location, yet that's what frequently occurs. Tuning them correctly often requires a more hands-on approach as well, something not everyone is comfortable with.
Multiples are routinely used to solve a problem; room modes which negatively impact the experience. Do you have that problem now? If there is just one main listening position you might not, in which case duals won't offer much value. The boominess you hear now may be due to low quality subwoofers, but it also may be related to room modes (where duals could help). Your ears can tell you a lot in that regard, but without measurement gear most of the critical information remains unknown.
Do you have more than one location where people sit? If so, do you have multiple areas where you can place subwoofers? The answer to those questions will provide key decision points for you. Sometimes one really nice subwoofer is better than multiple average ones, sometimes it's not.