I wish I could upvote this comment more. I have to run over a hundred instances of MongoDB to equal the performance I could get from ten Postgres boxes, but now we have such massive lock-in on MongoDB we can't easily refactor.
Any wrapper would likely be worse than just keeping MongoDB.
We have the exact same issues with Mongo-specific queries. Even worse, our indexing regime (because we store relational data in Mongo) demands that we stick with Mongo 1.8, so all the comparative goodness of 2.x is lost.
And just this week, it was determined that there won't be any migration from Mongo to something-SQL in the near future, because it'd be too risky to attempt to translate a schema that's half foreign keys, half embedded JSON.