I've got 6" over WG front and the 41mm tubes have never shown any sign of flexing or torquing in the hardest of turns, slow or fast. The handling is rock solid, don't think the braces are really worth it IMHO.
IMO, no. I put one on my '97 WG - long skinny forks. Didn't make any differance that I could tell. Maybe I just don't ride hard enough. But I got it on a clearance sale for $50, so I wasn't too upset.
It all depends on the bike and how hard it get's ridden. A fork brace made a world of difference on my Ironhead with it's skinny 35mm fork legs and single disk brake. No more shimmy on hard braking. It also took out most of the wobble in hard cornering.