From reading about this a long time there is one problem when people throw about the usual hull speed formula - finess ratio. The formula in its common usage has a ratio that is kind of universally used to describe a wide but common variety of displacement shapes. Unless you apply a different ratio to it you don't get anything meaningful for catamarans. This was in a Sailing World article a long time ago (I probably still have it somewhere) that addressed this very question. Using different finess ratios that more closely reflected the shape of the hulls the hull speed numbers became more reasonable, at least when they stay in displacement mode. When you put up a big headsail and get the newer designs honking on the flat section of the hulls things are a little different. But just because a cat may go faster than the traditional hull speed equation says it should does not necessarily mean it is planing, although it may ultimately do that as well.