The game appears to use "C:\Program Files (x86)\Steam\userdata\<steamid>\553850\remote\input_settings.config" to store input settings in the below format:
settings = {
mouse_acceleration = 0
mouse_smoothing_frames = 0
mouse_sensitivity = [
0.049999997019767761
0.049999997019767761
]
}
I tested it by manually adjusting both values to 0.01 and camera movement is significantly slower.