I am waiting for my new Seascape 24 to arrive. Had a J-24 for a while. OP if your races are all windward leeward, and in fairly light wind, it's going to be tough for those guys. Years ago when I was sailing in your area there were a lot of light south wind days...8-12 knots. Actually the water line length of a J-24 is about 20 feet (see slant bow, narrower stern) whereas this boat wl is 24, so it's hull speed will be about half a knot faster. However in light conditions, wetted surface is important, and it has more wetted surface so it will take longer to get up to full hull speed.
Off wind, at 12 knots they should be a little faster than the J24 and should be able to plane on a reach but not too deep. The more wind, the deeper they can sail, and still plane. But on windward leewards, it's a pretty tough race because if you can't plane, you are just a displacement boat without an articulating pole. Because of the articulating pole, the spinnaker boats can sail much closer to dead down while keeping good air flow on their sails. At 8-10, these boats will only plane on a beam or close reach. In light moderate conditions a J-24 to weather can use a 150 genoa up wind very effectively. But heavier upwind, the Seascape should be faster because it has a longer water line, in heavier conditions wetted surface is only a small part of the drag that slows the boat down. Very light conditions, racing is very much an issue of keeping good air flow on the sails. Off the wind, the whole question is wind speed and wind angle. It's because below 90 degrees actual, apparent wind speed drops the deeper and faster you go. At 12 they should be able to plane but not deep. So it would be right that in light moderate wind conditions especially on a windward leeward course, these boats might not be any faster than a J 24. If that's your normal conditions you should provide this to your rating committee. In more wind, or on triangle courses, this boat will perform to its current rating. On a reach at 15 knots this boat will absolutely run away from a J-24.