With the Taipan F16 uni, I was much faster VMG sitting on the tramp, always keeping the hull up while sailing deep as possible. Keeping the hull up while sailing deep is really key factor, and I imagine that's just as true with the nacra 17. Trapping downwind on the Taipan did not offer a net gain because you could only push it so hard until you ran out of bow volume. For tactical reasons, I have occasionally found it helpful to trap w/spinnaker; eg if the course has a reach and the wind is light enough to carry the spinnaker.