As I reported in bug area - mirror tool is not mirroring supports.
If you won`t fix mirroring supports then maybe you will do new feature inside preview of GCode to mirror GCode with all same data only with opposite direction. It would be only in X axis. It`s far enough.
I think it would be very helpfull if someone prints both sides of object (as me). For now it`s needed to rebuild whole support structure once again on mirrored side. It`s waste of precious time IMHO.