(I must at this point therefore state that the following is a theory based upon the evidence at hand, whilst taking into account some of the differentials Red Bull use over other teams whilst using past examples of how teams have looked to exploit certain design aspects of the car) I will also point out that their Splitter/T-Tray design is not new and the team have been using it for some time
Above: A close up the RB9's T-Tray shows the team drilling holes into the upper surface of the device to dissipate the heat generated
So why endanger your race with the chance of the plank bottoming out? An onward effect of running a Splitter that can contact with the ground with high rake is the Front Wing is placed closer to the track and therefore also operates in more 'Ground Effect'
So how could the heating of the T-Tray aid in increasing downforce?
Even without the aerodynamic advantage talked about above that could be gleaned from the heating of the top side of the splitter, we must also consider that using it as a heatsink alone has an advantage. The transfer of the heat via the lower skids to the upper element of course still allows the Skid Block / Splitter to run closer to the ground as heat is transferred away from the plank. Therefore meaning it can contact the ground a little more often without the fear of it failing the test after the race.
Lastly we also have to consider the stay (the metal element, in the case of Red Bull's latest iteration of the Splitter) that creates rigidity between the forward top face of the Splitter and the underside of the Chassis. Although we can't see any heat transference in the images to the stay it is plausible that this too is conducting heat from the lower element and buckling in the process (Either via lateral flexion or via negative thermal expansion). This too would allow more deflection from the splitter when it comes into contact with the ground further enabling the car's Front Wing to run in closer proximity to the ground.
The effects of such a practice are of course consistent with the teams desire to increase rake enabling a Front Wing that runs closer to the ground, whilst also increasing the potential for expansion of the area available in the Diffuser.