1/2" copper verses 1/2" pex have the same external dimensions, but the internal dimensions are different pex pipe walls are thicker, thus smaller and lower flow capacity. Any restriction to the tub spout will often be enough restriction to cause it to slowly backup in the showerhead riser, and dribble out there as well. Plus, there isn't a good way to attach any of the tub spouts to pex. They typically are either a slip on or a screw on connection. You can't anchor the pex, so your spout would move and the connection would be a problem. Pex up to the valve is okay.