Why do we care how hot the manifold gets? I can understand the original question as it might indicate a fault but it's not like the manifold is going to melt.
The heat shielding I can see as useful to keep heat out of the inlet duct or stop the exhaust heating up the back of the radiator or generally keeping under bonnet temperature lower but I can't see it aiding cooling the exhaust, but I may not understand what type of dividing plate Paul means.
On an aesthetic note I wouldn't want to cover up your pretty gizfab manifold.