Why would you port from cocos2d-x to cocos2d-iPhone ? cocos2d-x already support iOS as a target platform, there is absolutely no need to port the full game to cocos2d-iPhone.
If you want to implement some platform specific features (like Game Center), just use a few wrappers for those features, and keep the rest as a single C++ code base. Porting to another language is always long and tricky due to the difference between languages, regardless of which ones exactly.
On the other hand porting from cocos2d-iPhone to cocos2dx is a one time investment that is worth it if you need cross-platform. From my experience, developping a single app in different languages for different platforms is a major time sink for little to no added value.