I just pruchased an '86 R80GS and the headlight is anemic, scary even.
Rick Jones, from Motorrad Elektrik, who sells the excellent alternators, points in a can and superb starters for airheads, and more, told me that he ordered an H4 HID bulb kit for his R80GS and it DOES fit inside the headlight assembly. I had previously tried to fit an H4 HID inside the headlight bucket of my R90S and there is no room for it.
Rick said such a bulb will fit the R80GS. He ordered one from his local dealer who got it from Tucker Rocky, but they are available all over the internet.
Key is finding one that has a small ballast and is good quality. I am elated to find out this information.
Rick's stuff is here: