Stunning Speed is additive with similar mods. For example, a Sicarus with a max rank Stunning Speed and Sure Shot will have a 6% × (1 + 30% + 90%) = 13.2% status chance.
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%.