I did that test some time ago :-)
Here I compared two 'identical' white XR-E's, one on an aluminium board with dielectric layer, one on a MT-Noctigon. On copper, at 2A, the output is 10% more than on aluminium, the current at maximum output shifts from 2A to 2.4A, on copper at 2.4A you get 20% more output than on aluminium at 2A.
So, even if the red XR-E is mounted on copper, it will be still way outperformed by the red XP-E