Main Menu: Is it better to subdivide it into different scenes?

It depends on how your menu is set up. The system I use is an enumeration depicting all the different menus, like main, options, graphics, sound, etc. and a simple switch in OnGUI to render each menu only if it is active. This I find is incredibly light on performance. I get over 600 FPS with it and there are still a few models and particle systems in the background.

I can give you a simple example:

 enum Menu = {Main, Options, Graphics, Sound}
 Menu currentMenu = Menu.Main;
 
 void OnGUI ()
 {
     switch (currentMenu)
     {
         case Menu.Main:
             RenderMain ();
             break;
         case Menu.Options:
             RenderOptions ();
             break;
         case Menu.Graphics:
             RenderGraphics ();
             break;
         case Menu.Sound:
             RenderSound ();
             break;
     }
 }
 
 void RenderMain ()
 {
     //Build your main menu here.

     if (/*options button is clicked*/)
     {
         currentMenu = Menu.Options;
     }
 }
 
 void RenderOptions ()
 {
     //Build your options menu here.
 }
 
 etc. etc.

So in each Render___ method, you simply build your menu in code, set the buttons to change the enum, and it will automatically change the menu!