Star Wars-like assault map project

I want to create an space assault map with vehicles and jetpacks for players, with a big ship as base for defenders and turrets protecting the outside.

The problem is the performance.

At the beginning it'd be a big open space with a big ship that has turrets and that kills fps.

Here is a very rough concept of what i want (sorry its a .map file): http://dl.dropbox.com/u/54224716/star-wars.map

Obviously its only a concept, don't think the ship would be that way, i just did that in some minutes for testing

I just want to know if it worths trying to create something like that since the engine doesn't like a lot open places.

If it does worth it, how can i improve the fps?
would anyone be interested in helping me with the map?

This is something I've thought of recently too. Like one of the Assault maps on UT2004. Keeping the outside and inside of the ship totally separate might help by building them in separate areas of the map and linking with teleporters for going inside and out.