It's probably possible to increase the framerate of the 4k ultra resolution to 25 if I can figure out how to change the input modes. I can increase the output resolution of the normal 4k mode but it just upscales the 3840x2160 source image to what I put.
Thank you!!! BTW, I have settled for 3840x2880 in 4K ultra 24 fps ("writel 0xA0637AD4 0x0B400F00"), instead of 4000x2700, 3840x2880 has proper 4:3 aspect ratio so it is easier to do lens correction and it still has some additional vertical resolution for cropping/panning during editing. I can also mix with 1080p 120 fps when needing slow motion (5x in 24 fps) and render all in 24 fps without stutter. One interesting observation, using 120 Mbps, the frame bitrate per mega pixel is about the same for 4K30, 4K24 and 1080p120 (120 Mbps / (resolution_in_MP * frame_rate)):
4K30: 0.48 Mbps / MP
4K24 (3840x2880): 0.45 Mbps / MP
1080p120: 0.48 Mbps / MP
The quality of a captured frame cropped from 3840x2880 (4k24 4:3 mod) to 3840x2160 is virtually the same as the 4K30 frame which is confirmed by my tests.