Powder coaring them would also increase their diameter and maybe give a better fit in the bore (fill the wad colum better for alignment).
The powder coating ones are .682 to .683 dia and didn't notice any difference. I have shot hundreds of lee 1oz slugs but these are more accurate for me anyway.
The skirt on these doesn't seem to be thin, not sure about the original lyman 525gr but after shooting around 25 on sunday i didn notice any go flying way off target.