I also find this bit in the official documentation
"Masking and Channel Artifacts
Because of a quirk with DirectX, the Green Channel of a texture will often offer the best compression. If any of your masks suffer greatly from compression artifacts, first try placing the information into the Green Channel to see if that helps. If that does not fix the issue, then try using the Alpha channel to store the information."
So, I guess is settled after all? 
If you have more relevant infos please let me know!