Unless you removed the restrictor (some can be easily, some can't), each shower head is mandated to have a max of 2.5gpm. But, 1/2" pex might be hard pressed to provide 5gpm. The ID of pex is considerably smaller than that of copper in the same 'size' (which is the OD, not the ID). This is because copper is much stronger, so it can be thinner than pex. What's strange to some is that 1/2" copper is unlikely to be 1/2" ID and it is 5/8" OD, although it is much closer to 1/2" than pex.