Hmmm, I get different results in winISD. My numbers show the IXL in a 4 ft^3 enclosure (Qtc 0.796) is down by 4 dB as compared to the UXL in the same enclosure (Qtc 0.582).
I calculated at Xmax for each driver, with LT (f0=35.71; Q0=0.58; fp=20.00; Qp=0.71).
If one looks at the maximum SPL tab which does not factor in EQ, the IXL isn't far behind the UXL at 20 Hz (-2 dB) and exceeds the UXL from 35 Hz and above (by less than 1 dB). Depending on amount of EQ I can see where the are pretty much the same. With a different enclosure size this difference may be even less. $400 is a lot of cash, 3 for less than price of 2. 4 of the IXL's would be pretty cool.