I had this conundrum too. My car ran hotter as soon as I fitted a taller more efficient aluminium rad but it had this under tray on before and after.
My theory is its down to the thermal management on the modern Duratec engine (and maybe the Sigma too) that appears to be quite precarious. These engines are different to the older engines as they have their thermostats located at the bottom of the engine on the coolant input rather than more traditionally at the top on the cylinder head output.
On an engine with the stat on the coolant input the engine temp is regulated by a thermostat with 2 inputs. One input comes from the cooler coolant exiting the rad and the other is the hot engine by-pass coolant path. The stat response to the combine temp of both coolant paths.
So if you add a bigger more efficient rad the engine has to run hotter to raise the by-pass coolant temp to offset the cooling improvement in order to open the stat and regulate engine temp i.e. the stat will open later.
