a sphere of, uh, 10 or 12 edges (can't remember exactly but we are talking of about 120 polys) would look fine even at medium/large sizes, when smoothed.
Probably you could get a similar effect with a 2d ani (someone should try..) except for you will not see the sphere passing through ships. The point is that it is necessary to find a way of giving the impression of something getting close to you, until you get hitted.
The 2d shockwave on random plane at the same time of this sphere could give this impression...maybe...and a particle system could work too for the same purpose
A collision algorythm for shocwaves is already present in original fs2, so probably it would be just a matter of increasing the bouncing.
