Andrew,
When the RX7 oil cooler thermostat warms up, it moves a spring loaded disk against the bypass port outlet which tries to force all the oil to flow through the core. If the pressure differential across the core exceeds about 50 psi, the spring loaded disk
lifts and allows oil to bypass the core. The oil thermostat not closing properly and oil bypassing due to pressure relief would be identical in terms of how the oil passes through the system. I don't know if a Cosmo oil cooler has the same function.
Using the temperature of the oil returning to the engine from the oil cooler as the criteria for oil cooling effectiveness, this oil temperature will be higher when hot oil bypassing in the cooler is mixed with the cooler oil that has passed through the
core. If the oil pressure difference across the core is less than 50 psi, all of the oil returning to the engine will have passed through the core and its temperature will be lower even though the amount of heat removed from the oil system could be the same
in both cases.
With the Fluidyne oil cooler the pressure drop through it didn't exceed 15 psi under any condition that I tested (up to 6500 RPM). The pressure drop through the RX7 cooler was about 80 psi when the front cover relief valve (1986 13B ) opened at 150 psi.
Oil passing through the cooler at this point was flowing both through the core and the thermostat relief passage. I cannot find any reference to a relief valve in the front cover of a 2004-2008 Renesis. If this is the case, then the oil pressure at the inlet
to an RX7 cooler must be far in excess of 150 psi under high RPM operation (over 2500 RPM).
Of course this discussion of oil flow is only part of the oil cooling story. Air flow through the core and the core construction are major factors, too.
Steve Boese