An ultralight that weighs 253.99999999 pounds is absolutely less than 254 pounds and so complies with the reg. In the real world the bigger issue is whether you will run afoul of the FAA. All the people in the FAA are pilots and they are focused on what is best for aviation in general. If someone is buzzing houses and flying over stadiums and crowds of people and making a nuisance of themselves and generating many complaining phone calls, They are concerned and something will happen especially if there is an accident. If you're the one doing it, whether you are over or under the weight limit you will likely be in trouble because your actions are ruining aviation for all other pilots. If you are a conscientious pilot that isn't flaunting the rules and isn't generating a bunch of angry phone calls then you aren't likely to ever have much contact with an FAA official. They are good people in my experience, and if treated with respect will return the favor. If you have modded your engine to have obscenely loud straight pipes and insist it is your right to fly a bunch of patterns really low over nearby neighborhoods, they are less likely to fight for your right to do that then they are to convince you that your actions are endangering the existence of the airport itself. If you don't modify your actions, they will not be your friend.