Enabling auto slow shutter allows you to use a 1/30 shutter speed, when using this in 1080p60 recording mode, what exactly does each of the 60 frames consist of? Is it a sample 1/30 of a second long taken every 1/30 second then written to 2 consecutive frames? Or is it a sample 1/30 of a second long taken every 1/60th of a second (the sample time would overlap, is that possible?) and then written to create a unique frame every 1/60th of a second?
The next question would be how the 28mbps bitrate is allocated to each frame. If it is duplicating two consecutive frames, is it using the same bitrate allocation as if there were 60 unique frames? or does it allocate most of the 28mbps bitrate (quality) into the first frame, and since there is no change, it's not required to waste the bitrate on the duplicate. Another option would be if it just used half the bitrate?
Any idea's or insight is welcome!

Just tried it, and does indeed just double every frame. So it is taking 30 images a second, then just showing each twice for 1/60 of a second. I can't speak to how this is compressed, because I don't know how I would go about checking that.

I wonder if the behavior is similar in the regular 60i modes, effectively making it 1080p30. I suppose you could shoot in both modes, and see if the increased bitrate/file size of the 60p mode gives better quality than the 60i mode at the 1/30 shutter speed. It might be hard to compare though without some repeatable motion in the scene though.

on another note, when using the time lapse recording (only available in 60i mode) I'm guessing you end up with effectively 30p content, unless it is taking a sample halfway between for the second field?