Do you guys have a problem with running this modal in "Full Screen"? (Top menu -> Render -> Display Mode -> Full Screen). It changes my layout to just Image Editor and destroys current layout completeley...

This one is obvious (old Blender version - Blender module was replaced by bpy module), but posting error messages will help indicating the problem in future. As for making this addon updated for current Blender version - surely doable, but there are many API changes since pre-2.5 version)

Well, not really, at least to my understanding how this works. Only thing that "ensures" white center is emission power - the more saturation you use, the more energy it has to emit to "overblow" color to white. The color information is still there though (in render result - right click on the white area to verify that and read RGB values) only on output image it is overblown to white. If you use hdr image - information is preserved. It can be used in postprocessing as well (for example glare node would actually still use that tinted color when used).