In my opinion a 9.4 sail will get a 122 going in that wind, but the 131 will hold it better and stay on planning speeds better. I have never use the 150 but I have a formula board that is about that same size a Mistral Devil and i think the difference will not be worth it.
Yesterday we had a graeat day here with less than 11 knots of wind and I was on my 131 with 8.0m sail an got it going and my friend was in a 133 with a 9.0 and he was going also. I wont buy a 150 for slalom. The reason is that the board was design just for crusing as the PWA rules dont allow a board that wide. I dont think it will be faster or any better than the 131. That is my opinion.