Not sure on SR20 but 4age thermo stat needs water flowing around it for it too open. Hence the bypass pipe, its the way that system is designed, i can only assume the SR20 is similar and if you have the thermo stat in backwards or only cool water on the important side that stat will never open, i suggest you seek if there's was/is a stock bypass that may have been avoided during the install, other then that you have a water flow issue which would be either pump related or the flow of the water thru the engine is severely disrupted preventing the water from flowing normally in and out of the radiator in a pressurized system.
The fact you have changed so much and still have issues lends me to say your water flow is interrupted and you haven't fixed it yet.
I know nothing about SR20 water system but you have a water flow problem i feel.