Thinking off the top of my head here...pv=nrt would probably only work for a static system. The entire cooling system has a pressure differential (consider the points directly before and directly after the water pump) which causes flow. Also, the volume of the system is fixed, and the volume of a given amount of air changes depending on pressure, no?
Regardless of the science behind it, I agree with TxZHP04 in post #13; all this modification will do is introduce a leak as the "mean system pressure" in the ET exceeds the limits of the lower cap. Changing the cap will not change the operating pressure of the system, as it simply bleeds excessive pressure (which only happens if the T gets too high, right?). Think more like underdrive pulley if you want to reduce the pressure...and have a head gasket handy.
Long story short: waste of time.