in tripods, how stable is stable?

Slightly off topic, but when you use a tripod where it might get wet or dirty, always extend the bottom section, and clean it before collapsing the tripod. It's easy to wipe off a leg, but cleaning a dirty joint usually requires disassembly. If the threads in a Gitzo tripod get dirty, you also need to clean with solvent and relubricate.

That's probably why I have a couple of bad scratches in my Gitzo legs. Dirt got in and I slide the legs up to collapse. It doesn;t affect operation. But it looks lousy.

Weight hanging from the tripod has limited effect on stability. If your tripod is planted on a soft surface, like grass, extra weight plants the feet more firmly on the ground. In extreme wind, this weight lowers the center of gravity, making the tripod less likely to tip over. In the latter case, spreading the legs to a wider angle helps more than any amount of added weight.

Mass hanging from the tripod is loosely coupled with the tripod itself, which is free to vibrate independently. Only mass firmly coupled to the camera is effective against vibrations generated by the environment or the camera itself or its operation. This is why a firm mounting mechanism, such as Arca-type clamps and plates, are so important to overall stability. Equally important is minimizing the height of the camera above the apex of the legs. Adherence to these and other details becomes obvious when you examine high-resolution images at the pixel level.

A good set of sticks, topped by a good head, can easily exceed $1500, which is what you expect to pay for a high quality lens too.