"I'm not sure of the spacing with the divider plates in the WP, but I did install them"
As was mentioned in the 2nd post by crazecars the clearance between the divider plate and water pump impeller is critical. http://www.wallaceracing.com/water-pump-mods.php If you didn't check it, there is a good chance that is your whole problem. You need to get that clearance as tight as possible without rubbing. If not your pump recirculates the coolant from the pump discharge back to the pump suction and reduces the coolant flow to the rad.
