I suggest to move from spritesmith to postcss-sprites:

• There is the same spritesmith module under the hood, so the quality of sprites will not changed
• It has useful filterBy and groupBy options, which makes him very flexible
• Out of the box makes sprites by retina prefixes (@2x, @3x, @4x), which resolves Сборка спрайта из файлов вида file@2x, file@3x #321, and also makes prefixes customizable
• We can get rid from sprite mixins by automatically adding needed rules to css file with onupdaterule hook or with separate plugin.
  That will help to resolve Убрать принудительный выбор css и html препроцессора.  #314, cause will work without any non-standard css functionality (ex. postcss-mixins)
• Also duplicates problem, mentioned in Генерация спрайтов из assets #257, when there are two images with the same name, but located in different directories, resolved by adding both images to the sprite, cause they are still different by location, which is right case, IMHO.

If this is ok, I can implement it