I picked up Quickdraw (-50% cooldown on weapon change) before logging for a while, and hopped back in to get some more XP before bed. It turns out that in play, having both the Quickdraw skill and the -50% cooldown on weapon swap perk stack directly, and instead of having 25% of the cooldown (as I expected), it completely refreshes cooldown on weapon change. This allows for some serious cheese with long CD weapons.

Example of said cheese: it takes about one second to swap between my main and off set, so in two seconds I can completely remove the 8 second CD for my Sniper Shot, allowing me to rattle them off far faster. In practice, I'm too lazy to do so when my left mouse attack on the Exitus is capable of knocking enemies prone (single target DPS tree skill), but it's... likely not how the feature was intended.

Edit: the Exitus sniper shot cooldown is 3 seconds native, but I have half a dozen weapons in storage with 8-15 second cooldown abilities, so the hyperbole isn't quite as much of a stretch as it could be.