I do not own a Nacra 20, but I have sailed one on TV.

At the last F18 event (the only one I've been in charge of measuring for) we measured 90% of the spinnakers, and three mains. We template-checked everything else. We had 30 or 40 boats and it took two people out of the fun for two days - we weren't measuring that whole time - much of it was waiting for people to bring their sails over. Boats were being weighed at another station.

In-house certification (something the F18s are pursuing internationally) would eliminate the need for measuring except in the event of a protest. It would seem to me that the 20s could be controlled fairly easily since you're only dealing with one loft, which has already demonstrated a concern and willingness to work with the class.