Today we have a rare heat wave in Ireland, so I thought it was a good time to see this effect for myself. I recorded the experiment on video. What I've got here are 2 x 15 watt peak (rated) panels charging a 40Ah sealed lead acid (SLA) battery and a Raspberry Pi 
At the start the panels were reading 55°C (read from a non-contact thermometer) and were delivering 710mA of current to the battery/Pi system. After pouring water to cool the panels the temperature dropped to 42°C and the current rose to 820mA. I'm going to use current as a measure of efficiency here . So that's about 14% efficiency increase for a 13°C drop in temperature.
 It's a Raspberry Pi Model A (the one without ethernet or inbuilt USB hub). I've got an external unpowered hub connected to the model A's sole USB socket and to that a WiPi WiFi dongle and a Arduino Leonardo. A 12/5V DC/DC car converter down converts the 12V to the 5V required by the Pi. The current draw from the Pi at the 12V end of the converter is 210mA.
 Measuring and extracting power from photovoltaics is not a trivial topic.