Hi Shadone, congratulations on a nice appearing and well thought out installation.
Although I've been in this hobby for decades and have seen a few "professional studio" installation pictures, I never gave a thought to soffit mounting speakers. Your old ATC link for Sax's studio did not work so a couple of questions.
Would the soffit installation work for any box speaker that has a flat front baffle and no rear radiation drivers or ports, such as classic acoustical suspension designs?
Does this concept involve constructing an opening just large enough to accommodate the speaker box, then installing the speaker flush with the face of the wall or room divider? Your pictures are quite dark so it is difficult to see details. From your description it sounds as if you had constructed a large room divider that contains all your electronics plus your main speakers.
Thanks for further info.