The shape that detects which avatars to prioritize rendering, seems to be roughly spherical and based on the FOV of the avatar somehow. As the camera is not spherical, it seems odd to choose a spherical algorithm for this, especially since half of that view should be unrendered.
Why does this matter? It's common that only 1/3 of visible avatars are actually rendered as non-impostors. The shape used for non-imposter calculation is putting several avatars, who cannot be seen, as fully rendered. This means 2/3 of my avatar rendering capability is being wasted on things I cant see at all!