I'd argue that Alacrity is great for Underlurker adds since it means we get in more ticks of Suppressive Fire, making them die faster, and therefore giving more uptime on the boss before Rage Storm. It also means we can burst harder because the Alacrity will make energy regen faster while hiding behind the rocks.

It just depends on how long the adds are up. I don't really want to do math, but to setup a basic equation here, you need to use X amount of GCDs with Y% (higher) alacrity in order to achieve a number greater than or equal to 1:

Z (Number of extra GCDs with Higher Alacrity) = X (GCDs) * Y (GCD Time reduced by Alacrity) - X (GCDs) * A (GCD time reduced with lower or no alacrity)

Ok I lied, I'll do some quick easy math because I'm a little more curious about it myself. I'll just use easy numbers. Let's say someone has 10% Alacrity in one build, and 2% in the other. We know that a GCD lasts 1.5 seconds. So GCD time reduced for each is .15 and .03 respectively. And I'm just winging it here, but for the sake of lazyness, let's just say that each Add phase on Underlurker is 10 GCDs long, or 15 seconds long. So let's do some math.

10 GCDs * .15 seconds - 10 * .03 = 1.5 - .3 = 1.2 GCDs gained by 10% alacrity from 2%.

So in actuality, people don't typically run with more than 8, so you'll get most of an extra Sweeping Gunfire off before the last group of adds die. Eh, so I was wrong. w/e. Math. So for

Sweeping Gunfire spam, yes, just spam the **** out of it. You'll get like 3 - 5 ticks extra with more alacrity. Plus like you said, it'll help with energy recovery behind the rocks.

*Feels proud to prove myself wrong using logic*