in addition to above advise a scan would be good to confirm that the ECU is also seeing the same temperature as the instrument cluster temperature gauge. The temperature sender unit is known to fail on all this generation of VAG cars and there are TWO sensors in the one housing, one feeds the temp gauge the other feeds the ECU. If the temp gauge is being feed the correct reading and is getting to 90 after a 5 mile run at greater than 40mph in our current weather conditions, cabin heater on cold, then the thermostat and the half of the temp sender unit feeding the gauge is working correctly, however the ECU may be getting a lower temp and as such will over fuel the engine resulting in lower than optimal mpg figures.
I will also ecko the advise about the plug in 'chip tunning' boxes being total garbage and should be removed immediatly, they are cheap for a reason in that they simply alter one or two values from sensors feeding the ECU, this in turn fools the ECU in to thinking the engine is in a different operating state and results in more fuel going into the engine, this may indeed give you a little more power but will also greatly increase black smoke and fuel consumption.
to be fair if you are getting 60mpg on short and medium length drives with constant 70mph + running then it is not that far away, the optimal speed seems to be about 65mph. on my own A2 which his correctly remapped and running a JDD gearbox on 16" A1 wheels and wided than optimal tyres at a motor way length journey at 65mph I get 72mpg, increase this to 70mph and this drops to around 68mpg
it also depends how many people are in the A2, more people and luggage less mpg, more noticeable on A/B roads than motorway journeys, but still a drop
Paul