How did I come up with 72Hz = 357.71 MHz? I don't remember.
72Hz is actually 377.63 MHz (3440x1440 CVT-RB2) which is over the 360MHz limit for 8bpc.
But anyway,... good news!!: I was able to find a way to switch from 10bpc (limited to 280MHz) to 8bpc to get up to 68Hz (355.93 MHz). Requires kernel extension. I guess that means SIP needs to be disabled?
...And for the first time ever on a Mac, 6bpc! Maybe. I need to somehow enable a timing that is greater than 360MHz to be sure (up to 480MHz). If I can do that, then I have 4:2:2 selection working for up to 540MHz so maybe I can get 4K60 from the HBR limited GTX 780M. The GTX 780M only supports DisplayPort 1.2 so there's no 4:2:0 option, plus I don't think the method I'm using is correct for newer GPU drivers which put the color depth/pixel format in each of the validated timing modes.
With 540MHz, 3440x1440 could do up to 101Hz if you don't mind chroma subsampling.
