I was passing a lorry on the N10 today while going past an entry. A motorcyclist pushed into the rather short space between the lorry I was passing n the one in front, then decided to enter the outside lane in front of me. He wobbled considerably in doing so n I thought he might go down. When in front of me I could see he had only one pannier, on the outside, and I'm guetjere was something pretty heavy in there to cause the wobble. A bit scary for me - maybe he's used to it.
You definitely need any weight of a load on a bike central but the most important thing is it shouldnt be able to move. In theory you should be able to carry a lot of weight if you think a Pillion might be up to 100kg. Kayak weights about 15kg and a full Gas bottle probably about the same. you dont notice it really.