My vote would be for the old steel rear bumpers. Waaaaaay less wrecking. I think the few radiators saved by the new plastic bumpers is far..far outweighed by the increased frequency of race-ending wrecks caused by unchecked aggression and a lack of real consequences.
Simply policing the racing more is an impossible task at any event of size. The new in vogue thing to do is just turn right into the guy next to you on the straightaways. Maybe all the bodywork should be breakaway?
Take the bumpers off the bumper cars, and they become just cars. Cars that we can race.