RE: Heavy or Light
From what I have seen of the European's speed attempts, they do use gravity assist (dive) to achieve their high speeds. Otherwise they would never get the engines on the pipe with the heavy props used. Since most of them tend to use the FAI guidelines, they are somewhat weight limited to 75 grams/dm^2. That puts it around 24.57 oz/ft^2
I reject the idea that a span limited airframe will be the fastest in their operating format, as the induced drag when pulling out of the dive will be significant and they do need to carry the speed through the traps which is a distance of 656 ft.