I had the same mast and sail two years ago, and I had the same problem. See:
Looking for power upwind I think that mast is too soft to use with the inf sail. If you sheet the main with the necessary tension, you bend the top of the mast extremely (even there is no cunnigham tension), and the sail will really flat at the top 1/3. No power on the main, poor speed and height. A bit less mast rotation is help, but that is not a perfect solution, as it is also depower the rig.
I have tried to setup my mast, tried more diamond tension, but independently to that the top part of the mast remain soft! I'm not an expert, maybe a professional guy can help you to find proper setup.
Now I sail an infusion, and the difference was clear at the first time I sailed the inf.
In your case I would think about to upgrade to wing mast.