Gearbox's math is always screwy so I don't know if they stack, as in simply adding one to the other.
However, on Maya I have 44% cooldown from the mod and 30% from quicken. I can tell a pretty big difference if my spec is missing either one. Then on top of those two I use a 39.4% cooldown relic, and I can still tell a large difference between the 3 together as opposed to just the mod and quicken together.
Right now I'm running a wildcat mod and only have quicken and the relic, and its very noticeable.