So I finally got some figures....
I measured the same trip (with and without the front spoiler) and here are the results;
Without/with: Total average consumption: 16,3/15,3 kWh, average speed: 78/69 km/h, duration: 33/38 min, outside temp: 10/12 degrees Celsius, total trip: 42/42 kms.
Then I also measured at a specific point during the trip traveling at a constant speed with the cruise control on:
Without/with: Consumption: 16,4/15,5 kWh, speed: 120/120 km/h, outside temp: 10t/12 degrees.
So there seem to be a 5% lower consumption in average and at 120 km/h constant speed with the spoiler.
If is true that I've only got 2 samples so far and that there was 2 degrees of temperature difference that could impact the measurements. Also the average figures samples are not exactly the same circumstances...shorter trip and higher speed in average...but seems that the spoiler do improve aerodynamics and lower the consumption to some extent...