Don't know anything about comparing watts since mine are only recorded on the trainer and not in the rear world.
But if you know your typical speed over similar terrain then compare it to previous years results which can often be found online. Bikereg has links to some of the past results and some clubs and organizers post them online. Also don't forget the race effort usually means a little more speed then typical; the group will push you if you can keep up to begin with and you'll push yourself hard either way.