Contrary to what some believe, this mod doesn't directly reduce reload times; it increases reload speed. The calculations are far different, and that can cause some confusion.
The calculation of reload time using reload speed as a variable is almost the same with melee charge speed, which is Base Reload Time/(1+ Reload Speed Bonus).
The 40% reload speed bonus at max rank equates to a ~29% reduction to reload times. You can calculate the reduction as 1-(1/(1+s)), where s is your total reload speed bonus.
Stacking this with Quickdraw should reduce reload times by 47%.
The 10% proc chance bonus from this mod is smaller than the 15% from Sure Shot, which is already insignificant with the current system. It is recommended to only equip this mod to maximize reload speed after already having Quickdraw equipped.