The reasoning for the absurdly high damage of the advanced acid is so it can breach the hull of a ship. According to UFOpaedia, explosions always do 50% damage to walls, while other weapons do from 25% to 75%, so 400 was the minimal to breach outer hulls with 100% efficiency (unless that formula has been changed at some point.)
While the damage output number makes it seems overpowered to the max on initial viewing, from my personal playing, I found that its effectiveness is fairly situational and that it works best as a sort of specialty weapon. It's extremely useful for breaching and taking down the big terror units and can be used pretty effectively against the more armored aliens like mutons and robin's gazers. However, the acid capsule gun's lack of autoshot and high TU cost (at 50% for snap and 70% for aimed, you will seldom ever get more than 1 shot per turn with it) generally makes the heavy plasma a much more effective means of taking down anything else.
That being said, it seems like people are having a variety of experiences with it. I'll try to think about the weapon's balancing some more.
useing it and one shot with advanced assed didnt open a ufo or maby its just the lukes ufos have hirer armor idont know
I'm not sure why that would be, it should be completely effective at taking out walls. I know that at least one version of Luke's Extended UFOs had extra walls on the outer parts of the ships to prevent you from seeing the shadows of interior objects. Could that have something to do with it?