There are several threads on the forum regarding these larger tires. The main advantage is using the stock wheels, but getting a larger-diameter tire, but apparently narrow enough to not cause interference issues with the stock pants.
If you want to go to anything larger, Richard Squires (works with Harmon Lange, and they make all of Van's round gear legs) can make you a set with a slightly longer (1") axle, and you have a lot more options for wheels and tires. I've been using 800 x 6 tires (no pants of course) on my 9 in the summers when I'm in northern BC/Yukon.
Greg