I am going to pass on buying any additional Sigma filters. On my first Sigma CP filter, the coating started to flake so I exchanged it for a replacement. This one is flaking as well despite mostly sitting in its original case. It's been used less than a dozen times since getting it. None of my Hoya or B+W filters are flaking despite having them for many, many years, and they are all stored in their own cases as well.
It makes one wonder that if there are coating issues with their filters if there might also be coating issues with their lenses as well.