Hey Doug
I install shower supplies with pex all the time and have never seen a difference in pressure. Thats probably because most shower heads are restricted to at least 2.5 gallons of water per minute. 1/2″ pex would most likely be pumping out 7-8 gallons per minute unrestriced
IF you plan to add multiple body sprays, that might be an area where you want to bump up to 3/4″ pex so that the fittings are 1/2″, or go copper all the way around
Hope this helps
steve
