There is no such thing as a high performance git anything. People at Google who use git (I used to be one of them) suffer from far worse performance than the people who just use perforce as such. In particular, the cost of a git gc, which invariably occurs right when you're on the verge of an epic hack, or fighting a gigantic outage, is unreasonable and perforce has no analogous periodic process.
We actually have an open source tool that allows you carve off parts of your Perforce server as Git repos. The repos can overlap in Perforce allowing you to share code seamlessly between different Git repos. You can generate new repos from existing code easily and can even generate shallow git repos that are usage for development.
It would be great if someone used that to write a high performance git server.
Could they?