The map does not show up in-game and starting it with a double-click terminates UT2004.exe with a generic runtime error. UT2004.log is empty. UnrealEd reports two missing packages. APVerIII_ST and APVerIV_ST. If you upload those packages I can start a little bot-stomp romp on the map.
The icy terrain should have it's ground layers scaled up a bit so they don't repeat as often. Open the TerrainInfo's properties, go to TerrainInfo -> Layers -> The layers you wish to scale up -> increase the value for UScale and VScale.
The fence around the generator is missing a post. I know the model only has a post on one side so it may be a bit of a hassle to make one for the left-over side, but I think it would be worth it.
Judging by the size of the actors the area seems fairly small. At one spawn the cliffls look as if they could be jumped over by players, but there is nothing behind them. There are many places where players can see behind the cliffs because they don't really fit.
I assume that the lighting and decoration here and in the elevator part of the map is WIP, so I will comment on that later, when a new version of the map is released. How are you going to handle the fluid surface? Will players be able to drain the lava sink?
I had to grin about the in-joke about Xan. I won't spoiler it here.
Some parts of the underground facility are so dark that bad lcd screens are going to have trouble displaying them correctly. If you would lightem up a little more you could eliminate the full-black and the dark parts wouldn't look so washed out. If you have a CRT or a good LCD you probably wouldn't notice such issues, but on bad lcd screens it looks horrible.
The lava-tubes (?) should emit a little light.
All in all I didn't like this area all that much because it's all just one cubic room/corridor after the other. If you could break up that shape a bit somehow it would look much, much better.
The island part seems fairly finished, so here are some thoughts on possible improvements:
In the island part the terrain is very steep on both sides of the volcano so the path looks like it's unpassable but the painted on path indicates it is traversable. You could make the paths a little less steep and maybe carve them in a tiny bit just so they look more convincing.
The wall-meshes you used have ugly streched textures at both of their ends, which isn't a problem as long as they end into another wall, a hill or a pillar, ... In your case some just end in the open so their end is visible. You should try to hide that somehow. The wall part that can be blown up isn't close enough to the other walls so you can look in and out of the compound on one side of it.
The mini-island where the defenders spawn has really cool stones on it but they don't match up with the terrain perfectly so one can see under them. It would probably be easier to have them match when the island was a little flatter and it would look more realistic too, maybe.
The blue underwater sand texture was a great idea but it should be scaled up a bit, like the ice textures on the terrain in the ice part.
You could add fords between some of the islands.
I'm not sure if people are going to notice in the heat of action, but the transition between the shallow water around the island and the deep water in the open sea around it is a bit too straight. It would look better if this was curvier. Not just small curves as you already got those, but big curves.
The lighting from the lava pit could be improved by making it brighter near the lava and darker in the rest of the cave.
What's the thing over the lava?
I'm not sure about overall the lighting. On the one hand, color and brightness fit well with the rest of the theme, on the other hand everything is lit very evenly. A little more pronounced shadows could help. It might be worth a try.
The general look of the area is great though. The skybox, fog-ring, distance-fog and choice of textures fit really well.
The performance problem on older machiens you talked about seems to be mostly due to the high-res terrains. Half the resolution should easily do the same job but would only have a quarter of the performance impact! Hourences (he made Rankin and countless other great maps) wrote a terrain tutorial and in it he said that some people opt for a terrain resolution with which one terrain-segment is about as big as an adrenalin pickup. While it isn't that high-res in your case, he said it is enough in most cases if a terrain-segment is as big as a Goliath tank, and your terrain is a lot finer than that!
I'd say, just finish your map if it runs fine on your pc and when you are done give it to someone with a weak machine and see how well it runs on it with a couple of bots. If it runs bad, decrease the terrain resolution.
If you have an xbox you should try to get a hold of a copy of Brute Force. It's not a great game (it's fun if you play it co-op with a friend and you don't take the game too serious) but its level-design reminded me a bit of the elevator area in your level. It has rocky lava levels and alien crystal planets in the later game and you used both elements in your elevator part. You might be able to get some good ideas from the game.