Can the advanced missile racks be engineered?
Yes, they can. They're one of two AX weapons that can be engineered. With G5 Sturdy, they're actually reasonably good against thargoids, though still not quite as good as Gauss, largely because of the very slow projectile speed, though also because they just do less damage, even post-engineering.
It's hidden under the hood, but when synthed for AX damage, they do just 30 damage instead of 50 like when using human ammo. That means that a G5 sturdy AMR does 12.5 DPS(including reloads), compared to around 16 for gauss.
There are some advantages, however. They take very little distro drain, for example, and generate much less heat than gauss. And while they're MUCH harder to hit with in normal circumstances, they do have splash to make up some of the difference.