In order to help with accurate shooting, I propose the following.
When using the sights (Default RMB), the sensitivity could be adjusted on the fly by use of the CTRL and Shift keys.
This could allow better control when aimin as follows.

• Press CTRL while using the sights - Reduced sensitivity, allowing more precise aiming.
• Press Shift while using the sights - Increased sensitivity, allowing faster traversing.

Without either being pressed it would work as now.

Addiitionally, if in the future a 'hold breath' feature becomes available, this could be tied to the CTRL key too.