Nice job on the pep of the helmets, they looked great. Even though you said one was warped, it still looked good.
On to the cutting of the faceplate question. When I made mine, I did all my bondo work first and then once I was happy with the way it looked I cut it. This way everything lines up perfectly when put back together, you dont have to worry about an edge being slightly higher or lower than the other due to more bondo on one surface. I made my cuts follow the lines that are in the helmet as details, that way when it was put together it just looked normal and not cut at all. If you want a pic of mine after the cut, let me know and I'll post it.
Looking good, keep up the amazing work.