I think the effects work on 'Q Who' still holds up! If they just rescan it in I think think it'll be rad. That episode just *works*. I don't think it needs to be massively improved.
There is much to be said for minimalism...especially in an episode like that where the stoic, terrifying solidarity of the Borg is communicated perfectly by how little effort they actually need to expend to control their prey. I think "flashing it up" would really undermine the quiet tension of this episode.
The only thing that might need to be improved is that Borg interior shot, where we pull back to see more of the hive. All it should really take is a small color correction though, to better blend together the set and painting.
But yeah, the actual Borg model and other effects in that episode hold up a lot better than I remembered.