Just to clarify the facts about the Mozzies platform weight. It wasn't always 55kg. It used to be a minumum of 65kg, and this was dropped to 55kg in the early 90s when foam, kevlar and carbon became available.
It sounds like this was a painful decision for the class as it meant there had to be heavyweight results for the old boats from then on, but even 10 years ago Jim Boyer was building platforms which needed 4kg or more of lead to bring them up to 55kg. These were well built, stiff boats that are still sailing today. There doesn't seem to be any need to build them heavier.

Someone mentioned about the fullness of the Cobra hulls as well, so I should mention that the Mozzie hulls built since the mid 90s are more buoyant than the earlier versions. This is better for sloop crews and they do tack a lot easier.

I guess one reason to specify platform weight is to ensure a certain amount of material goes into the hulls. If an overall weight is specified then someone could build super-light hulls so they can festoon the boat with equipment and still be down to minimum weight. With a platform weight specified, there is a weight advantage to having a "lean and clean" setup - makes for nicer looking boats I reckon!

Tim