I will keep my discussion to the Formula 18 weight. With the large sail plan you are looking at 330lbs. Anything less and you will be tying weights to the boat.
In So Cal there are quite a number of teams that are sailing with two guys on the boat. The weight ranges are anywhere between 330 and 370 for some very competitive teams.
In very light winds, less than about 6, There will be an advantage for the lighter teams. Whenever you are double trapezed I think the advantage shifts to the teams with muscle. Getting the spinnaker up/down fast and the crew running the mainsheet are a big help to the performance of the team.
Does an Infusion carry weight a bit better? Yes there is more volume in the boat. For the average weight teams I think that all the boats performances are all very close.
Later,
Dan DeLave