I don't think there was much wrong with either overtake. We are viewing the second overtake from much further back, so it's us that can't see around the bend, not the GTD driver. He's obviously got sufficient momentum to be able to overtake the second car, spending the minimum amount of time on the wrong side of the road and getting back in with still plenty of room before the oncoming car. If anything, I would say that he was probably traveling at speeds above the limit, but neither manoeuvre caused any of the other vehicles to deviate from their chosen course.
I used to get worked up by other drivers actions, but now I'm just happy to let them go. He overtook you and was gone. What's the problem? The further he gets away from you, the further he will be away IF he has an accident. Only then will you be impeded a little bit by having to slow and drive around the carnage.