The water will take the path of least resistance, so, in the manifold, there generally are valves for each branch to help you adjust the flow so each one effectively is equal and they all get even heat. If it is just an open manifold, unless each zone was identical (highly unlikely!) the overall heating would not be even. Keep in mind that the ID of pex is quite a bit smaller than the equivalent size of copper, so if you really need the same flow, you'd have to go to a bigger sized pex than the copper you are replacing. Now, breaking things up and using a manifold, each branch would need enough flow for that branch to be able to provide enough heat.