Those figures look a bit low for me, but I do remember I took about 30kgs of weights from my mates place in my top box and cornering was very fun (the weights were 5kgs each and were moving about when I turned!) and the front end was just a tad ‘vague’
Had about 120mph with my top box on this weekend (rush to gatwick…oops) and the front started to move about a bit, but nothing major, just didnt want to go much faster…
So 6.4kgs should be fine, try it and see, go easy but cant see too many issues with that.
Well just putting an empty topbox on will alter the bike’s handling slightly & the more you put in the more effect it’ll have.
But, the figure quoted is just the manufacturers covering their arse from a legal liability point of view, you should be fine with what you’re planning, if you’re concerned, then just try loading it gradually & see what effect it has.
If you’re just using it for commuting around London I really doubt if you’ll notice any change at all.
I’ve never met anyone who had a twisted back from wearing a rucksack in a m/c RTA. Go figure. When I’m in a wheelchair you have my permission to laugh at me.