I had never heard of this but I have downloaded and installed it out of interest.
For anyone else who would like to try it.
Download locations.
Autohotkey here
http://l.autohotkey.net/AutoHotkey_L_Install.exepseudo_full_screen.zip
This link should work if you are a member and logged in.
http://library.avsim.net/download.php?DLID=158165otherwise search using the filename.
First, run AutoHotkey_L_Install.exe as an administrator and install the top (32bit) choice in the installer menu.
Once you have done this, you should have generated a tile or icon like this
This is one in Windows 8.
Click on the tile or icon and you will see this screen.
Browse to the location of the fullscreen.ahk file you downloaded from Avsim, select the location for your .exe file below, in my case D:\Unzipped for both, and click on >Convert<
You will then find an .exe file wherever you chose to save it
Run that as an administrator and you will see this icon in your taskbar
Now run FSX (or P3D?) and you will be able to toggle to and from the full screen window as you wanted.
I can see no difference between this and full screen FSX achieved by Alt+Return but I have left it installed and keep my mind open.
By the way, it removes the taskbar without the need to set it to autohide and while it is running in the taskbar, pressing Win+F11 will have the same effect on any other active window.
Regards,
Nick.