I think it has more to do with the limited air flow with the NISMO bumper than the size of the oil cooler. I also think the heat wave over where I live in Texas plays a part in my high oil temps.
EDIT
And to answer your question about location, I think GTM located the Rotrex cooler and the oil cooler there as it was the best location that had empty space.
Here is a shot of the bumper cut out and all you see is IC.
Here is a shot of the driver side of the bumper where the Rotrex and oil cooler reside. They are effectively to the right of the IC in in a pocket of the bumper with little to no airflow.
I am going to add the SPAL fan and temperature controller and see what better temps I can obtain.
In addition I am probably going to order the Greddy turbo cast aluminum oil pan to see if it fits with the NISMO under tray. It has cooling fins and adds extra oil capacity. I just do not know if it will fit without cutting up something. I don't know yet. You can see the pan at the bottom of this photo. That is not my car but another forum member's I believe who had the Greddy turbo kit installed.