I gave the Hollow Board a little thought, and the biggest problem would be standing on it. It would need to withstand point loads of around 150kg or so... 85kg person coming down on one heel, would be approx equivalent loading of 150kg in 3 square inches, or about 100 pounds per square inch, which seems to be up around 700kPa !! That's a fair bit of force....
Although deck padding over another sheet layer, would help to dissipate / spread that point load....
EPS foam is about 16-35kg/m3
en.wikipedia.org/wiki/Polystyrene If you have a 250 litre down wind board, assume 10-20 litres is the case/carcass. Leaves say 235 litres of foam (approx) or 5-8kg of foam, but I would assume you would add back 3-6kg into the carcass (if hollow)?? I would not expect the weight saving to be massive....
I still like the thought of foam in the middle for an everyday durable board.... Even if a hollow board was 1-3% faster.... And I guess we would only be talking 1-3% faster....