Charlie,
I have been chasing the same problem for years... Last year I tried a BRP SkiDoo modular helmet, but the problem I ran into is my melon is so big, even the biggest helmet, I think I had a 3X, wasnt big enough, when I pulled the chin bar down, it smashed the face mask so tight it didnt seal and i fogged up anyway. But that doesnt mean that if you can wear and smaller size it wont work great for you! most people I have talked to that have one love it!
What I finally went to is and old school open face helmet with a flip down shield, and a NO FOG face mask, but I modified the NO FOG slightly, I added about and extra inch to the nose snorkel to get it below the shield, and added a little this piece of foam across the bridge piece the seals to your nose and cheeks (I got the foam from and old pair of snow goggles) Now I am fog free