If you sheet that as hard as you should be, you may break the boom. On the older style Nacra 20 (with the same boom), our boom would look like a banana when sailing upwind in 15-18. That was with the tack of the sail much closer to the sheet connection point than what you have had to do there.

You have a tricky situation with that sail. If you move the blocks forward, you lose the angle that puts some forward pressure on the boom and induces mast rotation. The F18 sail is probably an improvement but you'll likely need something custom (or modify that sail) to really get it dialed in....or beef up the boom.