Yes, a PR design allows for a smaller cabinet. It does not allow for greater SPL.
In a properly designed system they can be quite effective. I would not consider that Sub, however, to be a worthy investment, as an example. It isn't that it is poorly designed, per se, but it is barely an upgrade from what you have... if at all. By all accounts, below 40Hz that thing basically disappears behind distortion and lack of power.
I have nothing invested in recommending SVS to you (or any other good quality sub that might fit your criteria), other than trying to help (much like Pogre, Bill, Snake...) you achieve a better performing system.
In a greater than 5000'3 room, your current strategy is flawed. You don't seem to want to learn from our experience and advice. I am reminded of several sayings: one about repeating behavior and different results, and one about horses and water.
It is your system, your ears, and your ducats. Please do what you wish and enjoy your system!
Cheers!