Can you also add:
...to the "Device" section of your /etc/X11/xorg.conf file? You'll need to log out and back in again to test.
Option "RegistryDwords" "EnableBrightnessControl=1"
When logged back in, if brightness still isn't working can you try:
...to see if manually manpulating the brightness file will change the actual brightness.
echo 50 | sudo tee /sys/class/backlight/asus-nb-wmi/brightness
echo 75 | sudo tee /sys/class/backlight/asus-nb-wmi/brightness
And if that still doesn't work, can you then try acpi_osi="!Windows 2012"? The relevant lines in /etc/default/grub should look like this:
...and after reboot post back again:
for i in /sys/class/backlight/*; do echo $i; cat $i/brightness; cat $i/max_brightness; done