A pressure-balanced valve by itself doesn't affect the flow or pressure. It is sensitive to pressure changes, but if they are even on both sides, it does not affect the flow or water balance. If the rough-in valve has in-line shutoffs, are you sure they are both fully opened? On valves that use both the upper and lower outlet, the upper one designed for the shower has a smaller opening. This normally isn't an issue, as all new showerheads are restricted in flow. Did you for chance use pex to supply the valve verses copper? If you used the threaded inputs rather than soldering the pipes to the valve, it's possible there's some teflon tape blocking things, if you weren't careful.