The gold DVD's say 8x, while the silver DVD say 16x. Why wouldn't high grade gold be as good as silver in the writing speed?
Write speed also depends on the actual dye used, and the writer. The laser power should then be optimized for that medium, and is usually recorded in a lookup table (e.g. in the writer's firmware). Gold or silver (or alumin(i)um) is just part of the refection layer, but the dye is equally important.