Devon

As you've probably gathered from the responses here, this is a very difficult question to answer based on any real observation. Orphan boats like the Nacra 17 in it's various iterations aren't seen in major regattas in numbers against F18's. Nacra has made many different hull, mast and sail combinations in Europe, USA and your continent. Performance varies widely so it's hard to give an accurate view based on anything other than best guess on their general performance.
In my opinion, the weakness in the F18 performance curve is upwind in 7 to 11 knots. Below that, their large sail area and heavy weight help to maintain momentum and push through the lulls. Above that when they pop a hull and get two on the wire things start to work with less drag and the foils working as they should. So if you can pop a hull either with none or one on the wire in the 7 to 11 range you should be able to climb away to windward. Downwind would be much harder as the F18 can throw buckets of sail area at the problem and overcome any weight disadvantage. All of this assumes equal sailor skill of course.

Engineer

I know it's good to be able to blame the Seppos for all the ills of the world but in this case it's being a bit harsh. There have been different versions of the Nacra 17 in Europe and certainly in Australia of the few that have been produced over the years they have had a number of different hull, mast and sail combinations. Hardly SMOD and they don't fit a formula...................