I'm in complete agreement with Jake. In light air it's no big deal. In heavy air you can go just as fast but if the wind gets to 5-12 you are hosed. They fly a hull before you. They get single trapped while you sit on the hull. Then they get double trapped while you are single trapped. If you are going to race with a crew weight at or above 350 you need to sail a Nacra 20. 350 is the perfect weight for a Nacra 20. Plus you can sail the Nacra 20 with up to about 400 lbs and not be able to notice a disadvantage.
This is coming from a guy that owns a Hobie Tiger and a Nacra 20.
Mike Hill