On the contrary, most modern boats do have cored hulls. And yes, all polyester/epoxy materials absorb water (they are hygroscopic) and gain weight. The boat doesn't even need to be in the water to gain weight...moisture is drawn out of the air and over time, it adds up (increase is something like 1-3% of the weight of the resin). Only way to really combat this is to put in the boat in an arid climate for weeks/months to dry out.
I think you have a couple of issues to deal with on this question. Construction methods have changed a bit and now "newer" boats don't gain weight as badly as the older boats. The reason being is the core or lack of core. A H14/16/18 has a foam core that absorbs water and that's where the weight comes in. H20's, I20's, 6.0's and most the other newer designs don't have cored hulls and don't absorb as much water. I can say that my old H20 was still under class weight even as a 10 year old boat.
The issue with those boats becomes stiffness and that can make a difference.
To those that care enough to change hardware to remove weight, yes it matters and is important. Relative speed difference? Typically it's a game of inches out front but good sailing will overcome 5-10 lbs or more of boat weight.
