Well, I think really high poly asteroids should be single models, and be used as a feature of a game, for example for a destroyed planet you could use one enormous asteroid (model) surrounded by loads of little ones (field).
Id personally say max polys for a standard Asteroid is 300. This would mean that, for example, being within LOD0 range of 10 of them would still only use up 3000 polys, and the odds of being within LOD0 range with 10 asteroids on screen at once is pretty slim, so this shouldn't tax the game too much, and give the CPU/GCard more time for glows, specs, e-mapping etc
