I just came back from a south to north trip on the GR20 in early September. We took 10 days and while the double stage days we picked were long, they were not particularly difficult for the most part. I highly suggest doing the entire route if you have the time and I think we could have comfortable completed it in 8-9 days without feeling too rushed. The scenery was great and the south allows for a lot more solitude, at least at this time of year. The weather was also never too hot and we only had 3 rain days but never enough to hinder our progress or dampen our morale. I highly suggest to bring more cash with you than you think you will need. The huts are expensive! I also suggest camping or renting a tent, as it provides more solitude. I recommend south to north because you don't get stuck in large crowds and I felt like there was a lot more to do in Calvi post trip. The beaches in Porto-Vecchio are not as easily accessible.
For a detailed trip report with timing, elevation, and distance check out https://jamieroselevin.squarespace.com/adventure/2013/9/20/gr20-day-1 and scroll through to see the day by day including info on Porto-Vecchio and Calvi.
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