In the context of beach catamaran's (and high performance dinghies) which is what I beleive this thread is all about, Uffa Fox's comments still hold true, a lighter boat is always better than a heavier one.

Of course that does not mean that you can't design a lighter boat that is slower than a heavier boat, and you will be able to find examples where this does not hold true, but that is only because the lighter boat was not properly optimised.

Gareth