After looking at the schematics I also agree that it's most likely a ground. However, if it came with the retrofit kit, my electrical gut tells me that it may be intended to replace an existing ground wire to increase it's current carrying ability.
The drawings show the brown wire from the grip connectors going to the ground joint with the headlamp, then from there to the frame ground point under the fuel tank. So it could be possible that the new wire is intended to run from under the tank to that point were the headlamp and grip ground connections join.
If the bike was "pre-wired" for grips(depends on the model), this section of ground should already be the upgraded 1.5mm size.
The bikes not "pre-wired" for grip only used 1.0mm wire size from the headlight ground to the frame ground point(only large enough for headlight load).