At our last nationals there were about 40 boats, only a few were under weight, most were between 4-10kg over and quite a number 10+ honestly I don't know how much difference it makes on the race course, crew weights varied by more than 50 kilos with a few in the 200kg + category. On land I would prefer the boat to be as close to the 180 as possible (the old boat was 3.8 under and the current is 2.5) certainly makes pushing them around that little bit easier.