I don't know the exact weight difference, but I will say that just because a boat is built after 1984 does not necessarily mean it's lighter. I have a 1985 and a 1990 and also know someone with a 2003. The 1990 is by far the heaviest. The hulls are about 25lb more (each!) than the 1985 boat. Hobie dropped weight out of the 18 around 1985 to 1987 or so. Then in the late '80s, early '90s I think they went overboard and beefed the construction back up to compensate for some of the hull failures that were occurring as well as to make the boats stronger to handle the SX rigs. Then they eventually brought the boat weights back in line to the minimum weight. If you look inside the different years of boat, there are noticable differences to the constructions.
I can definitely feel the difference moving my heavier 1990 boat around on the beach compared to the 1985 or 2003 boat. As for performance, we still do fine racing it and I don't think it sails noticably different than the other boats. The 18 is pretty forgiving as far as weight goes. And the heaver boat is built like a tank, I imagine my 1990 hulls would last forever.