Where I work (game company with about 20 people) we solve this by versioning unimported assets separately (in svn, because that's what we used before git).

We bring them into the git repo after running them through our asset pipeline, which means that there are really never any merge conflicts for them and every checkout is a playable version of the game with art.

This reduces the load on git by a lot, though we still tend to have .git folders measured in the tens of gigabytes, which is getting up there.