Thats odd, cars are supposed to run cooler on the highway??? At what temp is your car running? If you have a temp gauge watch it and see when the car starts overheating on you. It will tell you a lot about your problem. Does the car overheat only when driving at continuous highway speeds or is it a matter of time? I second the suggestion of "burping" the cooling system. Air in the system will also cause the engine to overheat.
How far away did you install the divider plates? I know it will cause an overheating problem if they are too far away from the impellers of the water pump. You can also try and lessen the clearance to improve cooling: http://www.wallaceracing.com/water-pump-mods.php
