I haven't used one, but below are my thoughts on strenghening the chassis:
The frist step towards a solid structure is tying the front and rear portions of any car into the subframes. These are the overhanging sections where almost all flexing occurs. Adding Subframes results in the biggest increase because this essentially creates a 2/3 frame on the bottom of the car (with the remaining 1/3 of the car being the engine bay and rear
suspension after the frame rails stop.) Imagine the car divided into 3 boxes, the front box is the engine bay. The middle box is the passenger compartment, and the rear box is the rear
suspension / trunk section. In order for the chassis to be stable these 3 sections need to be rigidly attached to one another. In the Fox3 chassis, the sheetmetal is too thin and the spot welds are too few to effectivly tie all these areas together. This is what leads to the "loose" chassis.
So when I see a brace such as the one in the link you provided, I have to wonder what it really does. I mean, bracing the two shorter halves of the car together doesn't seem to do much since the car isn't too wide to begin with. I would also venture the comment that I don't see other companies out there with this same setup for other cars (nor the OEM's using it)
Things that help:
SFC's
Strut tower braces that tie into the firewall
Known to do little or nothing:
Rear shock tower braces
Just my thoughts though....
