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On most Canons (can not speak for the 1DX or 5DIII), ISO 50 is not a natural ISO. My understanding is, your sensor has a sensitivity range, for example, 100-6400. This is the pure ISO settings for your camera. The expansion settings allow you to extend that range down to 50 and up to 12K, 25k, etc. But instead of this being a natural ISO setting it takes a ISO 100/6400 sensor and adds a software algorithm to the mix to create the extended ISO settings. You could say it is similar to the film days when you pushed or pulled film. It can be nice, but it can also cause issues. I have used ISO 50 sparingly for well lit portraits and while colors and skin tone were real nice, the sharpness was just not as good as ISO 100.
As for not using ND filters, why not? In my film days I use to use ND gradient filters and ND filters all the time (not typically for sports, but sometimes). A ND gradient with a tobacco color shooting into the sun can give you some phenomenal sunset images.
What about a polarizer filter? Most of those take away some of the light that if you are shooting at 1/60th at ISO 100, you should be able to hit 1/30th with a CP filter, plus it would cut glare on windshields, and really make any blue skies pop.
I should add, most DSLR's have a native ISO. My classic 1D is ISO 200, my 1DMkII and 7D are 100. At these ISO you get the highest quality, sharpest possible image from the camera. Any other ISOs are not as sharp (for pixel peepers) as the native ISO.