I've now tried a 17, 16 and 15 on the F800. For off road and 2 up mountain road carving the 15 is great. For a combination of all things the 16 works fine and the 17 is great if you need to do a quick weekend 3000km round trip.
I dropped the gearing on one of my DRs for a trip up Cape York a couple of weeks back, and instantly regretted it as I spent about 95% of my time at freeway speeds on the dirt. Next trip, I'll take the smaller sprocket in the tool bag, and change it the night before the tricky sections of the trip. It's all of a 10 min job, if that. Same goes for the BMW. When faced with a few thousand kms of touring speed to get to the "good stuff", I think packing the spare sprocket is the smartest option.