I see lots of confusion here.
Except for the EXIF flag indicating exposure, the RAW file you'll get with ISO50 @ f/8 @ 1/400 will be exactly the same as the one you'll get at ISO100 @ f/8 @ 1/400. The only difference is that, if the camera's meter tells you that the proper exposure is ISO100 @ f/8 @ 1/400, it'll tell you that you should be exposing the ISO50 shot at f/8 @ 1/200. And, of course, the RAW developers are programmed to invisibly apply a stop of digital pull to the ISO50 shot.
There will be a difference between ISO50 @ f/8 @ 1/200 and ISO100 @ f/8 @ 1/400 -- and it's that difference that people almost always compare.
There won't be a difference between ISO50 @ f/8 @ 1/200 and ISO100 @ f/8 @ 1/200.
And, of course, there will be a difference in the RAW files between ISO100 @ f/8 @ 1/400 and ISO200 @ f/8 @ 1/400.
Hope that helps clear things up....
P.S. A very similar thing happens with highlight tone priority, except in the opposite direction. The RAW files for a given shutter and aperture will be identical between ISO100 and ISO200 + HTP, but the meter will be different. b&