This is one that could be argued/discussed forever. Some of the SAW record holding boats were actually on the heavy side so that kind of shows that it can be more about setup than dropping weight.
I rember reading about Glenn Quarles and his quest to blow the IMPBA "A" hydro record out of the proverbial water. He dropped every gram that he could and stayed awake nights thinking of ways to make his hydro lighter. He went as far as remonving the mounting tabs off the servos, drilling holes in the cases and not running a strut, only a brass tube as he knew where he needed it to be. The boat barely floated as there wasn't much of a hull or sponsons to keep it above water if it stalled or flipped.
Weight certainly can help in rough racing water conditions. It helps also if you want a boat that is good for more than one run.