Export Frames of 10 BPC Video File with ffmpeg
Try using:

ffmpeg -i "testinput.mxf" -vf scale=in_range=limited:out_range=full -pix_fmt rgba64 path/to/output/out%05d.tiff


instead.

For a reason that is slightly beyond my comprehension of color science (or maybe just something odd with ffmpeg), the output of the initial command was creating .tiff files in the "limited" (15-235) rather than "full" (0-255) RGB color range. I used the scopes in Premiere Pro and DaVinci Resolve to A-B between the first frame of the created .tiff sequence and the original .mxf and the differences became clear.

If you aren't dead set on using ffmpeg, I was also able to achieve a comparable result by importing the .mxf into a Resolve (version 15 in my case) timeline and then using the "Deliver" workspace, selecting the 16 Bit, RGB and .tiff options as the output and using either the 'Auto' or 'Full' range setting. If you go the Resolve route, you can queue up each .mxf file and just let them export until everything is complete. 

There are slight variations between the ffmpeg .tiff, the Resolve .tiff and the original .mxf but to my understanding the process of converting YUV (the original .mxf) to RGB (the .tiff sequence) generally isn't mathematically perfect. The files pass the "eyeball" test, however, and should work well unless the people you are working for have really strict delivery requirements. 

If you really want to go deep into it, the nice (and smart!) folks over at Doom9 forums can get you into AVISynth (which is really cool and powerful but also quite complicated) for very advanced conversions. 

Hope this helps!