I definitely agree with you, it seems nonsensical but I've had a quick hunt around and found other dumps of Dell DSDTs (eg
here)that have the same code present! It doesn't seem to be causing issues, so I think i'll leave it alone. I also used the dumps I found as a reference to fix SSDT-1, which compiles now and is included in my ACPI/patched.
As per your advice, I checked through all the SSDTs & DSDT for references to GFX0. Only DSDT & SSDT-5 made reference to GFX0/IGPU, besides the external references from refs.txt. I've also applied the brightness fix now to DSDT instead of the SSDT. Uploaded my current patched for those playing at home.
Things are running quite nicely but I think I just need to fix up the keyboard buttons for the brightness control, as i've noticed that if I use them it can cause the OS to hang for a second.
Still waiting on a wifi card shipment, but that's just one of the little disadvantages of being aussie.