The dynamics of a FFing crash cannot be duplicated with a RFing child, because of the differences in installation. Different belt paths, tether or not, the recline of the seat all add up.
It is true that a child in a rear end collision is at higher risk RFing than if they were FFing. It is also true that a child in a rear end collision is at higher risk in the back seat than in the front seat. The day you know you'll be rear ended, put your baby FFing in the front seat. In reality, we don't get to pick our collision. Rear end collisions are statistially rare, and lower energy than forward or side impacts. Rear end collisions are the least likely to cause serious injury or death. Frontal and side impacts are more common and more dangerous, so protect your children from the biggest, most likely threat.