Material double sided check dosent correct working

#1 I have the same problem, when double sided becomes checked specular/normal effect shrinks

#2 Normal and specular work from a direct light source striking the facing normal of a poly/plane/"textured surface" - if you make a texture double sided, then half of that polygon/"textured surfaced" isn't getting directly lit.

#3 We're aware of the problem and will be doing something different with the doublesided flag in a future release.

I suggest making proper double sided meshes and not depending on the material flag for that feature.