Spoiler alert: This post contains spoiler for those who haven't reached the 10-2 mission set.
So, I don't know if this has been pointed out before but the entire premise of the mission Dreams and Nightmares is that the core is opening and you and Vaelyn are entering it to find the Alpha Squad and see what Herex is up to with the Artifact.
When you play the King of Ashes or even regular Arcade you reach "The Core: The Path is Sealed" at Depth 28. However, in this one mission the entrance of the core is at Depth 25. Furthermore, the levels that are supposed to be within the core are at depths 26-29 when you would expect them to be 30+.
What exactly is going on here? Am I missing something or have I not discovered some insight that someone else has?
Also does anyone else experience tremendous FPS drops during that final battle of D&N?
I'd say the reason why it starts at depth 25 is purely technical. You usually move 1 depth lower with each elevator, so they organized the mission to roughly end around depth 30.
Other reason may be radiant drops. This way you need to make more progress before you get to those really lucrative boxes.
Finally, monsters' health (and presumably other mechanics as well) stop scaling after depth 30, so you could say that depth 30 is actually the final depth of the game.
But it's true they could do something so it looks more authentic. For example, in some levels your current depth simply displays as "???" and your gate map is disabled. There also seem to be a way to jump from deeper to shallower depth (this happens in Firestorm Citadel Shadow Lair). So there are ways to play around how you traverse through depths, but they are not used in Dreams & Nightmares at all for some reason.
Also, I remember that some people pointed out back when this mission came out that the Core doesn't actually open. The hatch is simply moved away a bit (to the south I think). You can notice some pipes being in different positions when you compare it to "The Core: The Path is Sealed".