I have a little experience doing a HUGE scale space sim. I ran into a few problems and here are the solutions I came up with:
-need to be able to control render order of objects
-I'm not sure how, but I've been told you can take direct control of D3D in GDK.
-The reason you need direct control is that the further away objects get from the camera, the less D3D (and GDK) can distinguish depth differences. This will cause something known as Z-Fighting even when the two objects are at very different ranges. For instance, a planet has a moon that is being half-way obscured by the planet. You will at different times see the moon in front and other times the moon behind.
-you need a coordinate system (and the math that goes with it) that can handle the vast expanse of space.
-the problem with normal float precision is that when you start using VERY large or VERY small numbers, the floats can't decide what the actual value is.
-this is discussed in the game design forum: http://forum.thegamecreators.com/?m=forum_view&t=199820&b=19
-I can provide an upload of what I have if you wish.
The fastest code is the code never written.