I always thought of them as shrubs. They're tough and can take pruning in stride but you're considering going so far back that shock comes into question. In that case prune as far back where you feel that shock won't come into question.

We used to prune regularly to keep the size in check, and more to the point to encourage the lovely new red leaves

id cut it back half then wait then keep trimmed as pamg sais . just give them a good feed and water afterwoulds wont hurt .