Restricting the flow rate through our racing radiators always resulted in the engine running cooler. Of course, at extreme RPM, it's possible the water pump impeller was cavitating and reducing its ability to move the coolant. I'm still thinking the most likely culprit is reduced radiator capacity. An aluminum radiator sinply can't transfer heat as efficiently as a copper core OEM style. If you have enough room, which it seems like you do, put a bigger radiator in there and solve the problem.
Jerry
