I'm using 4 similar 15" in a similar sized room and rarely get any serious excursion. I'm amazed your single driver has survived!
It is nice to be able to relax and use any bass level you like when watching a new film without worrying about cone excursion.
I worried for ages that something in my 4 x15" would break and then I wouldn't be able to replace my discontinued drivers. (Freight charges to Europe for 4 new 15-18" drivers is more than the cost of the drivers!)
Now I just use any level I feel like. (usually loud! ) We often enjoy 6 new hired films almost every weekend and just sit and grin at each other on the bassy moments.
I'd think in terms of 4 x 15" PE drivers. Or at last two drivers with serious Xmax and an option on two more. The better the bass gets the more you want without even realising it.
The more drivers you use the better the bass sound quality, the lower the distortion, the higher the headroom and the stronger the dynamic peaks. Each new driver is an insurance policy against disappointment.
BTW. A low wall dividing the room is invisible to a sub. It sees the whole room and tries to drive the whole room whether it likes it or not.