I've been offline a few days for xmas and trying to sneak in some plane work.
My primary data points on using foam for the leg fairings are from Checkoways' site, along with a few other builders who've documented the same. It seems now that this is becoming more accepted as a way to fix the problems with the standard documented installation procedure. Hopefully some flying with foam can comment on whether they'd do it again.
Here's checkoway's comments. It looks like he flew with hollow fairings and then opted for the foam after some issues.
checkoway's comments/installation on foam
As you know the instructions just have you clamp the fairing at the top. The upper/lower intersection fairings help prevent fore/aft/rotational movement of the fairing. They aren't attached. They just hug it, but that's all van's requires for standard install. The wooden rod and reinforcement at the tongue are more recommendations, but I think using great stuff foam would be a substitute for both of those recommendations. It helps reduce stress on the clamp, and I would think it would help with shimmy. It certainly provides much better stabilization for fore/aft/rotational tendencies.
As noted, the downside of the foam is that you can't just take the fairing off in a minute and put it right back on. If maintenance is required, I figure I can do anything from cut it open to dissolving the foam out (messy!) to cutting it off and ordering a new fairing.
One thing I can tell you about great foam is be religious with the gloves. This stuff doesn't come off period. I always start off a messy project being good about gloves, but I never learn. Somehow by the end of it, my hands and clothes become victims.
The only thing that bugs me about the use of great foam is that it is flammable, and it's heat rating is below that of dot4 brake fluid. If I didn't know that many others were flying just fine with the foam, this would weigh more heavily on my mind.
I'll post some pics of my wheel pant installation soon.