I recently bought a secondhand Polaroid SX-70 Polasonic type 2 camera which takes the Impossible SX-70 films. Because I work for Transcontinenta , the Benelux Importer of Impossible Project, I was able to try it out with some expired films
I noticed that the older the film the more washed out the colors and the more overexposed it looks.
You have to "waist" two pictures of your film pack to see the best minus correction for your aged film.
The Impossible film is the most sensitive to light during the first 10 seconds. The use of a frog tongue helps to improve the contrast and diminish overexposed prints. Use the dark slide to get the picture and put it somewhere dark for 40 minutes. Only the new films from June 2016 onwards need only 20 minutes and give already a "preview" image after 2 minutes.
I also noticed that there is a great flexibility in the Impossible SX-70 final print when taken a picture of with my iPhone 6+. I shoot with HDR on and edit then in Snapseed.