For the frame rate, I tested 23.976 (by selecting 1080p24 in Canon settings), but MLVFS reports the recorded footage to be at a weird 23.972, so I'm getting back to good ol' 25. I could test the FPS override setting in ML, though.
At 1280x720, recording flag stays yellow, so I guess that means I'm losing some frames. Now I go with 1440x602 (a ratio of 2.39, which is a popular ratio in movie theaters nowadays, and still often referred inaccurately as 2.35). With this setting, the recording flag becomes green after 15 or 20 seconds of recording.
Two years ago, I shot in H264 1280x720 at 50 fps, because I wanted to do some slow motion, but I guess you wanna talk about high fps with raw recording? Then in order to do that 50 fps, I guess one would have to lower the resolution, the limiting factor being the SD card writing speed. E.g. my 1440x602 would become 1024x428 or 992x416.
Upscaling does not seem to be a good practice to me.