The specs for that model say it has a 5.0 GPM flow rate. Most modern shower heads are 2.5 GPM.
My plumber is here now and I've been discussing the pressure problem with him. He removed the Kohler Pressure Balancing Valve, took a close look at it and told me he doesn't see how an adequate supply of water could possibly flow through such small holes. I told him that that model has a 5.0 GPM flow rate and he's baffled. Could it really be that this valve is the source of the water flow problem? He does agree, BTW, that the 1/2" CPVC could be problematic, especially since it branches into two showers.