I don't know of any statistics you will find about anti-rebound. In general, the better a carseat is coupled to a vehicle, the more benefit the occupant will gain as the vehicle absorbs more of the crash energy. Better coupling also tends to reduce head excursion and the chance the occupant can strike part of the interior. In the case of rebound, the concern is that the infant seat could rebound all the way back into the vehicle seat or pillar in a crash. My understanding is that this is rare and only attributed to minor and moderate injuries in a few cases.
On the other hand, there really should be now downside, either. Many years ago, there was some concern about rear-facing tethers being a possible issue for small infants due to the potential for increased forces on the neck and spine during rebound. Rebound bar systems tend to stop more gradually and control this energy better, which may be why you no longer see rear facing tethers on infant seats. I never did see any indication that the increased forces posed any real world risk and some had the opinion that it was still better to reduce rebound in severe crashes than to allow the seat to potentially strike something in the vehicle.
Also consider that today, most infant seats allow you to leave the handle in the UP position while in the vehicle. That is a type of rebound management, too, though probably less effective than a bar that begins in contact with the vehicle seat.