Don't know why the thread is closed, but I can tell you that the NP RS:Slalom (MKII or MKIII) sails are, according to me, no good sails in overpowered conditions. They have not enough twist and are really very trim-sensible. The NP RS:Racing sails are MUCH better for this. I've opened a few months ago a thread about it: NP RS:Slalom <-> RS:Racing.
You should go for a REAL formula / top-end racing sail. A formual sail is nothing else than a top Racing sail with size bigger than 10m2.
I have myself a Gun Mega XR 10.9m2 sail which is VERY good formula sail in overpowered conditions AND a lot cheaper than the NP RS:Racing.