The boss' enrage timer starts when he's pulled, not when you personally start attacking. Spending 7-8 seconds to buff yourself at the beginning of a pull is wasted dps which may not necessarily be made up with a stronger burst, especially when it can get tight with dps check.
As for re-loading the ammo thing, you may or may not want to delay your reload/quick reload usage depending on the fight. If I don't use my reload right after lead shot in A3S, it won't be up in time before the first wash away (and therefore I cannot prep my 1-2-1 shots to use on the go)