Cheers for answering
Regarding marine equipment I'll just tell you that COs and XOs will rarely use a SG, Incendiary ammo can go dry pretty quick and once you spent the mag you'll be finding yourself ditching it for a normal magazine instead, SL armor isn’t game changing although it’s true that it protects a bit more than standard ammo, I don’t get why you're calling Smartgunners Sergeants, they're corporals.
Anyways I cannot talk much about SG armor, as I haven’t tested if they are better or worse, Specialists get armor related to their kit (SADAR and GL have armor that protects them from explosives, Sniper lightweight armor worse than light armor, pyro protection to fire, etc.) It’s not like Xenos can create explosions or fires (yes I know acid is a thing, but it still hurts when you're pyro)
Smartgunners job is to put up suppressing fire, they're not meant to kill as it takes quite the time to put down a defender, fending off xenos is their main job, grossly comparing xeno caste with marine roles isn't valid in my opinion, as both mechanic and role wise are too different.
Xeno gameplay is more about trapping the enemy and buy time for age upgrades as well luring and capturing marines who go ahead of themselves early on.