Insert Clip Alpha - best practices?

For some time now I am focusing on the Insert module and I noticed that the output of the module have pretty bad quality when using Insert Clip with Alpha (i.e. single PNG image with white strokes on transparent background).
Even if using Erode Alpha or increasing Anti-aliasing, there is a black outline around the the opaque parts of the Insert Clip.
What is the compositing mode for the Alpha Chanel (straight, premultiplied)?
Any guidance for maximum output quality in such case will be appreciated.
I am using MochaPro 2.6.2