I think we do know it would help. If your car can't physically see the car 1/4 mile ahead, but gets info that it has slammed on its brakes, thats very useful.

If you've ever driven in bumper to bumper traffic on a highway you know how invaluable this info would be, even to a human driver.

Plus info about road conditions (icy spots ahead!), etc..

Chris Urmson, back when he headed Google's self driving car effort, said car to car communication was unnecessary. The car has to have good enough sensors and decision-making to deal with all the things that aren't communicating, from pedestrians to deer.

I agree that you shouldn't assume any information will necessarily improve performance, but I feel that, naively, inter-car communication would have tons of useful applications.

If anything else, during the transition to fully-automated, it would be wonderful if human drivers were able to visualize the intent of the automated vehicles around them. To know that a car is going to merge into my lane in 50m, or is attempting a U-turn, is so much more useful than simply seeing a turn signal.