If you only sail windward/leward courses, a high aspect main and a spin are all you need. Throw in a reaching mark and, in my experience, the boats with large jibs do a lot better.

In the places where I sail, light winds are also very shifty. In those conditions I can't see how you could use a spinnaker. First of all, there's often not enough air to fill the spin, and secondly they're too hard to control when your point of sail can go from upwind to downwind several times on the way to a mark. A hooter or screacher would do a lot better in those conditions.

If you can't use the spinnaker effectively, an Inter 20 should be a good bit faster than an F-18 based on sail area to weight ratio. Just counting main and jib, the Nacra F-18 has 220 ft^2 of sail area and is 385 lbs. The I-20 has 246 ft^2 of sail area and is 390 lbs.

I disagree with you that area in the jib is better than area in the main in very light air. Among the group I sail with, the dominant boats in very light air are an A-cat, and my own. I've got a 5.8 with a heavy dacron square-top with lots of shape. That sail will pull me forward even when the jib is limp and lifeless.