It's going to be far better than any of the other common weapons, besides maybe a Katana, and it's going to be best on the Infiltrator due to tac cloak (~ 1900 damage a shot). Whereas a Soldier wielding an Avenger *needs* some kind of piercing to efficiently (more or less) damage an armored enemy, the Mantis (and Katana...I guess) hits hard enough that it can function well enough without any mods/equipment. Other classes can of course use the Mantis/Katana too, but the Infiltrator - as far as straight weapon damage goes - does it better.
To sum up: I didn't mean to be snide, I guess I was just opining that the HI does fine on Gold - especially in a squad.
While I don't typically bother with the maths in this game, you're not factoring in the damage reduction from armor, which I believe is %50 (? someone feel free to correct me). With no armor mitigation of any type on the Mantis, you are now looking at 950 damage, which is quite pitiful vs any Gold boss, especially factoring in the ROF ... I wouldn't consider that Gold viable in the slightest.
To kill a Gold Atlas like that, you would probably have to visit an ammo box before taking it down.