I ran a radiator shop for a few years...and fixed many a cooling problem.

A restrictor does not help cooling...at best you gain nothing. Think of it this way..Unrestricted, the water will flow through the radiator more times per hour...increasing cooling. Restricted, it goes through the rad less times per hour, reducing cooling. Yeah it might spend a bit longer time in the radiator, but thats nothing compared to the water flowing through the radiator several times more per hour.

Read this:

http://www.stewartcomponents.com/tech_tips/Tech_Tips_3.htm