If Christ is indeed a proper name to Paul (and this point is debatable), the translation will largely depend on your opinion (title? or name?). That's why I say err on the side of caution. It's not as clear cut as if he had said "Jesus" or "our Lord Jesus." It is not an issue with validity the rule itself, but a perceived ambiguity surrounding "Christ," if it is a proper name or not. Only Paul can know that.
So Peter can write a sentence and think he Is writing Christ is God.
Paul writing the exact same words has the meaning of Christ and God as separate, dual addressing.
The Rule is only for mind-reading Fools!