I don't play OW really anymore, but I play other FPS often (mostly Planetside 2)... I used to play with a sensitivity of about 17.78cm/7in for a 360, I did well enough, usually maintained a K/D over 2.0... I dropped my sensitivity down to 55.88cm/22in, my headshot/kill ratio went from about 10% to 40% and I started averaging K/D in the 4.0 range. Low is good, it took getting used to, but once I was adjusted it was FAR better. The problem with high sensitivity is even when you are used to it you will still overadjust because it is simply too fast to make fine, smooth, small adjustments.
Arm for large movements, wrist for fine adjustments, headshots for days.