I know it’s an old post but better late than never.
I regularly get 20km/litre with a mix of city and country cruising. Usually 2 up.
my bike is a standard 98 model 1100 with the exception of staintune slip ons.
I don’t ride particularly hard but also dont worry about economy. managed to get 420km on 21litres yesterday.
Considering the size and weight of the St the fuel economy is pretty amazing.