Great build, You may want to try one of the thermostatic valves for the incoming water. More expensive but they adjust by temp not pressure.
http://www.homedepot.com/p/Honeywell...US-1/202264710
Many shower and combination tub/shower mixing valves in todays homes use a pressure balanced design. Unfortunately, pressure balanced type valves do not automatically control outlet temperatures, nor do they automatically limit outlet temperatures should the supply temperatures dramatically increase or decrease. Such limitations are potentially dangerous. Further, to prevent users from adjusting the valve beyond a safe operating temperature, pressure balanced valves require setting of the limit stop by the installer. In addition, to accommodate future changes in the hot or cold water temperature, the limit stop would need to be re-set. By contrast, Watts Thermostatic Mixing Valves offer precise control of the outlet temperatures, no matter how the supply temperatures or pressures vary over time.
http://www.watts.com/pages/learnAbou...ves.asp?catId=