Don't get hung up on the size of the driver. That design couples the driver to the box very well. Anyhow two of those would outperform one 15" any day. I use four 10" drivers in my room and they shake the floor and seats. The UM series are really for sealed application and take a huge amount of power and Eq.
The drivers I have selected make for a much better designs then any of those drivers. The RSS 390HO only achieves an F3 of 33 Hz, so not really suitable.
Unfortunately Dayton changed the design of the RSS 460 HO. I did a design for the original driver, and a number of members built them, and it was an absolute killer sub. The I find that they did a redesign and the T/S parameters changed significantly. They really mucked the driver up, and I can not recommend it.
I must apologize, I linked the wrong sub design on my above post. The designs were one above the other and I clicked on the wrong file. Here is the design I should have sent you. I have also added another design from my file. So you now have designs for a 12" and a 15" unit. The 12" will produce 109 db. at 20 Hz, and the 15" 111 db. at 20 Hz. Both have very nice FRs and will be excellent subs.