As RedFlag pointed out it's not longer easy to produce a below weight hull due to panel surface forward. Actually the first clak 40 sailed by Nicolas Destais was heavier than the minimum.
About adding weight to the keel. Class 40 limits righting moment, so d pending where you add weight to the...
It's to prevent spray from shrouds and caperail rail going in the water. Probably adds righting moment when heeled over. Some class 40 ,add a tarp in the area to prevent the annoying spray going on the cockpit.
It's actually the old paprec arkea that won route du Rhum with yoann richome. If you...