In case anyone else has been looking for information on this, I found the following.
Someone at nano-reef.com used a PAR meter to measure light from a Panorama Pro module in a 28 gallon biocube, and came up with
1 12K White/445nm Blue Module - 55 PAR at the bottom of a 28 Gallon Biocube
1 12K White/445nm Blue & 1 445nm Blue - 85 PAR at the bottom
These were the only actual PAR readings I could find.
28 Gallon biocubes are 22" high, although I'm not sure if that measurement includes the height of the hood. (Source: http://www.f3images.com/IMD/UserManuals/JB7137.pdf
Assuming that the output does not change dramatically between the 12k/445nm LED's and the 8000K LED's, it seems reasonable to assume that one Panorama Pro strip would give at least 50-60 PAR at the bottom of a 18" tank. I understand this is considered solidly medium light (PAR vs Distance, T5, T12, PC - New Chart
This also seems to generally agree with an Ecoxotic sales rep's claim about the Pro modules giving out about 40% more light than the old 13w versions. (Source: http://www.youtube.com/watch?v=HAr45ri8Rp4
The claim I reference is at 3:45. The pitch on Pro modules starts at 3:22.) I am terrible at math, but I think 55 PAR is about 153% of 36 PAR.
On the other hand, a guide provided by Ecoxotic to Drs Foster and Smith recommends 2 Pro modules and 4 stunner strips for freshwater planted aquariums 18-29 gallons. (Source: http://www.drsfostersmith.com/pic/article.cfm?aid=2998
) I guess this would bring the bottom up into the high light range in an 18" tank.
I also emailed Ecoxotic directly about this, but I'm still waiting on a reply. Right now it seems like one Pro module would be sufficient to grow low-mid light plants on the bottom of an 18" tank. I'm not sure if 50-60 PAR is enough to keep foreground plants low and carpeting, though.