Is it safe to ride a racing road bike in very wet weather?
They're fine. I live outside of Vancouver so I'm already riding in wet a few days a week some weeks. Just this morning I was coming down the backside of a climb at 70km and they were totally secure.

Since the tires are so narrow they don't suffer from hydroplaning. The biggest thing to worry about is painted lines and manhole covers (or other metal covers on the road). Both are very slick when wet. I've always been careful but I know a number of other riders that have gone down cornering hard on both types of surfaces.

You're right to leave yourself extra room to brake. Also don't forget to account for reduced visibility for both you and cars.