Interesting - I think I used a little less ginger beer as I was using lowball glasses, but I quite liked how the spicy bite of the ginger played into the tequila, giving it almost a smoky sensation. I had to take a second to recall I did use tequila and not mezcal. Lime comes through, but the cassis flavor seems a mere accent - though it is likely also countering the tartness of the lime as well.
I'm with TV that cassis=lime, otherwise it's overpowered.
I had 45\20\20 with mezcal topped with FeverTree Light Ginger ale.
The excellent ginger ale helps make it. mezcal
The mezcal stands out despite the oomph of the lime and the FeverTree. Amazing.