Much on this on the forum already about the subject in general, e.g current threads:
However I don’t recall there being much note of diagonal setups on this forum, though it has been suggested a few times when people have had problems with awkward shaped rooms, though I don’t recall any subsequent feedback. I have heard one myself, which was forced by the awkward L shaped room and the competing demands of furniture, and it sounded surprisingly good though it wasn’t a full diagonal setup,.
Acoustically setting up a rectangular room diagonally would suffer from the lack of left-right symmetry which might negate any benefit, but for a square room that wouldn’t be the case, and I guess it could work well, reducing some of the room problems. In practical terms I suspect that for most people having also to use the room as a domestic living room, arranging it around its diagonal might ‘feel’ wrong, while sitting facing a corner might not work aesthetically as well as facing a wall with pictures or windows etc, but it certainly would seem worth trying.