Thanks to the diligent detective work of the So Cal Abarth Club and the So Cal NASIOC Chapter, there's a very high probability it's this guy:
https://www.instagram.com/vics_sti/
Still no legible plate in video, but the 3 Abarth that he almost ran over to execute that unsafe pass on Angeles Crest Highway shared some screen grabs, plus with what we can see from the front of the car, have matched quite a few details.
Same offset license plate, something mounted in the center of the front license plate location. Yellow fogs. Front splitter and side skirt similar.
The So Cal Abarth Club member whom the STI tried to stuff into the mountain had dash cam footage too.
Third brake light under the diffuser, similar stickers on the bottom left bumper. Too bad the plates aren't legible to compare, but we're almost certain this is the guy.
Turns out he posted a video of a "run" with a Porsche Sunday morning on Angeles Crest Highway shortly before we were coming down from Newcomb's Ranch on his Instagram feed.
https://www.instagram.com/p/Bges-jpFDVf/
As well as another post about how one should "stay in your lane,"
Turns out the guy's a hypocrite and a habitual double yellow lane crosser.
I know it's a tightly knit car enthusiast community. Please spread the word so EVERYONE having fun on local mountain roads (Angeles Crest, Ojai Highway, PCH, etc, that this guy frequents) be careful and be aware of this douchebag.