G3D's lighting model is physically based, so there's no reason to
think that "1 W/m^2" is a good value to avoid saturating or
underexposing the image. I think that it happens to work out so that
is actually a reasonable value, but I'm not sure. It might just be
that your textures are too dark or that you don't have a bright enough
environment map.
I've switch to almost always using spot lights instead of directional
lights. There I have some intuition for the values (which are in
watts) and they tend to act more predictably in hybrid renderers. For
a space game, the lighting has to be all fake anyway (things are too
bright or too dark in real outer space) so what you're doing is
probably the best design.