Hi gr5.
I was impressed with the simplicity of your experiment and followed it for a simple test of a new extrusion system that I intend to attach to a UM and just wanted to thank you for providing the information.
I also thought you may be interested in the test results, we tried 5kg which was not a problem, so we tried 15kg's. The 1.75 PLA filament snapped at just over 10kg's before the extruder had any chance of slipping.
Link to video demonstrating test procedure.
Regards
Arnold