Remember, we do not care if the data is on disk or not, just that if it does make it to disk that it preserves the atomic property we were after. All that needs to happen is for the rename not to be reordered in front of the write. That hardly restricts performance.
As for a new API, yes, that'd be great. There are doubtless other situations where it would be useful to be able to constrain write re-ordering. For example for writes within a single file if we're implementing a persistent tree structure where the ordering is important to provide atomicity in the face of system failure.
Having a nice new API does not mean that the obvious cases that app writers have been using for ages are wrong. We should just insert the obvious write barriers in those cases.