I get the whole idea of hyperbole and metaphor. I understand wanting to use that to explain away Christ's words on enemy love and self-sacrifice here in Matthew 5. But I just can't buy this method of explaining away the teaching for several reasons.
1) The repetition of the expectation undermines the notion of hyperbole. If Jesus said "it's better to cut off your right hand than burn in hell," then followed it up a little later with, "no, seriously guys, cut off your right hand," then a little later talked about it again, it would make me wonder whether this was really hyperbole. The repetition and building of a concept undermine this way of explaining away the teaching.
2) Christ literally exemplified what he taught here. In fact, his life exemplified a much stronger ethic than he even taught. Jesus wasn't just slapped on the cheek. He was beaten and murdered yet did no violence in return.
3) Go check out the early church quotes and study the church fathers and the early martyrs. Their words and lives embody the ethic of Matthew five taken at face value.
4) Pragmatically speaking, if my goal is to advance the gospel of Christ, then his words in Matthew five make much more sense if taken at face value. I know this because when I look at examples like Jim Elliot, MLK, the Amish, and other individuals or groups who embody non-violence at times, the gospel comes to life and is validated like it isn't when violence is used. Violence often invalidates a message.
5) Christ's words in the isolation of this one passage may be argued as metaphor. But taken in the broader context of apostolic teaching, biblical examples of non-violence, and the rest of the cumulative case for pacifism, it's hard to take Christ's words here on non-violence as metaphor.
This is obviously a very short response to this rebuttal. John Howard Yoder in "The Politics of Jesus" does a fantastic job of laying out the case for why Christ isn't using metaphor in his teaching on this. I recommend the book, or you can check out my somewhat shorter synopsis of it here.