This may not be of much help, but I have an 08 iS 111, and weigh 80 kg. The largest sail I have used is a 7.6 race sail (Maui Sails TR4). It take about 12 knots to get planing with pumping\in fresh water and a Techtonics Tomcat 42. Yesterday, I hit 29.5 knots on my gps in 17-19 knots of wind. Not overpowered, but well powered and comfortable. Over 20 knots of wind and the water gets pretty rough for 111.
For you on 9.0 sail, I would think that you could get planing in 10 knots with pumping on the 122. A little less in salt water. The 9.0 would be a bit large over 15 knots in my opinion. For the 122, I would think anything over 20 knots would be a pretty rough ride, but that depends on how much chop you will sail in.
In the 15 - 20 knot range, an 8.5 - 7.5 should be plenty of sail and then a 6.5 over 20.
Someone with a 122 will probably be of more help. The iS is fun and exciting to sail, I think you will enjoy it.
I race, but mostly formula. I haven't raced the iS and likely won't since I rarely find a slalom race. Just freesailing.