I really love this map.
I won't pretend to know more about how it plays than you and your extensive testing, so I won't give feedback on that.
Instead, I'll give feedback in an area I think I'm pretty good at, detailing. I went through your map, here's a bunch of thoughts. Most of this shouldn't make optimisation any worse.
(img)
Lighting is really important, especially for important entrances like this. Unless you want it to be a sneaky entrance, in which case leave it.
(img)
(img)
The facade of your base in general is really stark and boring. Add some more crap too it to make it look more than a clean building. Pipes, vents, cables, etc. Take a look at the top and outside of gorge's red base and you'll see how it's done.
(img)
This area felt pretty empty in general, it looks like the area people would store random crap, but it's just empty. No one ever notices the little mining boxes in official maps unless they're part of gameplay, because they're used everywhere to clutter up areas without distracting the viewer too much.
(img)
Use smoothing groups on this surface to get nice lighting. At the moment it's faceted and ugly. Read more about how to use them here:
https://developer.valvesoftware.com/wiki/Smoothing_groups and
https://developer.valvesoftware.com/wiki/Hammer_Smoothing_Groups_Dialog
(img)
This whole area isn't very nice at all. The water seam with the skybox is huge, it looks like you've used two different water textures completely. The trees at the back make it look like a lake, which completely devalues the lighthouse even more than it already is (turned on during the day?).
(img)
It doesn't look like you've compiled with proper shadow settings at all. use -staticproppolys and -staticproplighting in the vrad command dialog to make nicer shadows. It will increase compile time and filesize a bit, but will look a thousand times better. Also a good area for clutter like I said earlier.
(img)
More of the lack of good shadows again. It looks the worst on the pine trees, these trees look basically fullbright, very tacky in my eyes. Additionally, the mineshaft lift tower looks a bit out of place.
This is what they'll look like with the proper compile options, I think you'll agree, much better.
(img)
(img)
Your map has quite a few of these little nooks that look like routes because they are lit. Remember before how I was talking about how powerful lighting was? I kept trying to go into these because lighting led me to believe they were important.
(img)
I really dislike the texturing on this building. The second texture has a normal map and the first doesn't, they look completely wrong together. I didn't even notice the wood border at first either. Use the top brick texture for the entire thing, well prooves how amazing that texture is on large faces anyway.
(img)
Do what you did on second, strew rocks and that around on the ground to help break it up. a rock or two on the wall might help too.
(img)
Utilising this lantern and actually putting a light there would improve this area a lot - the health is important, it should be lit well. It would also give an excuse for those boxes to be lit.
(img)
I do not like this wood texture. It's far too noisy for a big surface. Try using one of the barnblitz or goldrush wood walls instead. I do like the beam and tirmmings of this area though.
(img)
The final room in general feels really empty visually. You should be able to spend a lot of detail here because areaportals should seal it really well from the outside, so you may as well go all out.
I don't really like process visually for the most part, but it is a good example for my point with it's final.
(img)
It employs so many pipes and ibeams (as well as bight lighting) that final looks really interesting. Yours at the moment looks really empty in comparison.
(img)
This is another example, you could easy place some more windows up there to let more lighting in which would serve the double purpose of breaking up the tall, blank walls. Pipes are great to use too. And you could easily stand to place more overlays around, they're super cheap (and can be turned off in the configs anwyay) so use them! A giant team logo would be good (maybe with a backing of some other material though), and some propaganda posters. Look at process again for examples.
(img)
You could easily afford to increase the lightmap scale on this one wall too, it would make those shadows a lot nicer.
I also suggested adding a spotlight to fake more env lighting, it would brighten up the upper areas of the room in general and make it live up more to the maps name!!
Bonus:
(img)
You might not like this, you might. Just letting you know.
Apologies for my terrible handwriting, Haven't really done much recently, let alone on a tablet