I can't give any info on how well these act as feet for heavy racks. I have a hifi setup on shelving and I am out of space. I got a brainstorm to use these feet to give some airspace between gear. These worked perfectly, the 1U device I wanted to stack had screws on the bottom panel. I centered the screws so they rest in the holes and the flat part sits on the device under it.
For the price I don't think you can go wrong.