Its a gift
I didn't find any current ratings for the 280 but 228 sounds like a gift. Most boat of this size have a base rating less than 200. Even adding 12 for nonspin I don't 228 seems high. Most club don't make adjustments for lack of a genoa generally wouldn't make a difference in a rating. Also, isn't the 280 designed to sail with a big roach in the main and a relatively small genoa? You should do very well. Don't be surprised if the race committee wants to reevaluate your rating fairly soon.