You said with the latest nightly build you are getting 1856x928 ? I don't understand , I don't even see a option to choose 1856x928 in that nightly build. Am I missing something here?
Go to the Movie Tab , with the Big wheel selector High Light the "Raw Video(MLV)" Tab, Press the "Picture Profile" button to enter the submenu.
Now you will see a list of option to adjust , high light "Resolution" use the small turn wheel by the shutter button to select frame size etc...
or press the "Start Button" to enter submenu.
I am contemplating if it is even worth switching builds to shoot at 1880. You think it will make much of a difference?
Not really, It depends on your project needs and Workflow
If you are looking for just the largest Frame Resolution (A.R.1.85:1-1880x1016, A.R.16x9-1880x1058) then .raw Version 1.0 is the one to use.
Or even in 3x Crop mode with original format ".raw" you can get 2048x1024 for around 60 seconds ! or 2048x930 continuous, but No Audio which is not to big of a problem if you has a clapper board & master the audio to a exterior PCM recorder etc...
If you need Audio for e.g."Run & Gun"style video or your workflow needs the extra metadata that .mlv generates(TimeCode, ISO, Expose, Lens Info etc..) for Grading programs like Blackmagic Resolve or Adobe SpeedGrade and the smaller resolution 1872,1856 @ A.R. of 2:1 is acceptable then Version 2.0 .mlv is for you

Also- if there an option in ML to change Framerate?
In Movie Tab high light "FPS override" press "picture profile" button to enter the submenu .
Normally I have My 5d2 set to "Exact FPS" @ 23.976 from 24 fps
The highest frame rate you can push the 5d2 is 35 fps as long as the Canon menu is set to 30P
or as low as 0.150 fps but never tried that low , have heard of people record at 0.250 fps with out problems.