I use the CU 180 on my Mk III and my Mk II. They are actually made of Gunmetal and sound slightly more alive than copper. The performance of the Mk II improves substantially with the CU 180 mat. Maybe I have been lucky but to date no sonic issues with the added weight.
I also have used a copper mat on my Garrard 301 beneath the stock rubber mat with good results. I believe this adds substantial mass to the platter in essence damping the chatter of the original aluminum platter and lowering the noise floor. I have listened to the Garrard without the stock rubber mat only the copper mat and prefer it with both. Once again no problem with the extra weight on the Garrard.
I would like to try a CU 180 on the Garrard, but have not to date tried one
because the CU 180 does not fit the spindle of the Garrard and needs modifying.
All the turntables are mounted in Porter panzerholst bases.