Bell, The 8lb cap would limit the whole system to 8lb max pressure irregardless if small tank cap was 16lb. If it is a closed system (as described) suction or discharge doesn't matter it will equalize pressure and find the weakest point to exit . The purpose of the pressure is to raise the boiling point of the coolant, coolant has a higher boiling point than water and for every 1lb pressure raises the boiling point by 3degrees.(antifreeze raises it a little) Internal hot spots (flow dead ends) in the block/heads may boil without proper pressure, that's why most cars run 16lb for a margin of safety) modern cars fans don't kick in until well over the boiling point.
Panteras use relatively low temp sensors in the radiator to regulate fans which is not the hottest point of the system. The coolant around the cylinder walls/heads is usually much hotter, especially during hot soak.
(From your description I would think your system regularly discharges small amounts of fluids due to the 8lb cap pressure , block hot spots and heat soak. I would think it would eventually seek some consistent level but change pretty drastically according to heat load.
Just my opinion, your mileage may vary.