Hi Scott,
It's cool to see you over in the DIY HT side of things. Now that I have a house, I'll have to hit you up for some of those new IB 18's with the 2 layer coil.
Not to hijack this thread but I think one of the better comparison of XBL^2 stuff were the Blueprint 01 series that Scott did. While the resulting paraders of the drivers were different (and I like the higher BL and Mms originals) the XBL^2 versions did have a good deal lower 2nd harmonic distortion and (in my opinion) a cleaner upper bass sound. There are some measurements here by Monte K showing this:
http://www.mfk-projects.com/woofer_data.htm#Blue
You c9uld also do a similar comparison between the Blueprint 03 series / RE HC to a Brahma.
I think the biggest issues in using licensed technologies is (in public or private / for better or worse) being associated with the other company doing the licensing. Also, while Scott may have some input on this, for 95% of applications, even with cost not as much of an object, a "standard" overhung is the way to go, especially if you need high BL / power handling.