the site.
it's been a long time, but i read some very interesting articles on the magnitude of improvement to be gained by extra weight under the tripod center post. if you shoot 30 seconds and a mild breeze is blowing you have a potential problem that the extra anchoring will reduce.
i agree with the other posts that state running your tests with faster shutter speeds, and encourage you to consider the "extra weight trick"
the reports i read some time ago were aimed at showing the difference on medium format "mirror slap" moving the camera and the results were pretty astonishing by adding weight under the tripod.
it's the final print that 