Rondidon,
It sounds like your on your way to an optimized project.
You should not have needed to reduce from 1600 to 500, that is, if you are refering to verticies.
1600 verticies should not be that big, some of mine are about 10000.

BTW, I had noticed that your nexus number is below 200. That's really good.
In the documentation it has been written that we should try to keep at or below that number.
That is not possable in my huge lanscapes. I am lucky to stay below 400.

About your "Sky boxes"... have you thought about getting away from those big things and use a system like "Mysty Mood" offers?
I have spent a great deal of time experimenting with sky boxes and sky domes.
I really like the Mysty Mood system... If you haven't checked it out, I suggest that you do...

Keep pounding the keys...
And post some shots for us to view so we can offer you our thoughts...