In addition to what Maedze said, another caveat to the whole "race car driver" argument is that they ALSO have a helmet on which is restrained/attached, such that their HEAD can not fly forward and place too much strain on their neck...kids in 5 pt harnesses do not have that. My understanding is that in a frontal crash in a 5 pt harness, there is extreme stress placed on the neck, because the torso is completely restrained by the harness, such that the head flies forward, putting all of the force of the crash on the neck we are trying so hard to protect. In a booster, the side of the body opposite where the shoulder belt is, also rides forward a bit, as does the entire upper spine, etc, and thus, the forces placed upon the neck are actually LESS than in a 5 pt harness. That is why some countries, like Sweden, do not FF harness, but use a booster right after rearfacing, at about age 5. And to me, it makes PERFECT sense that this would be true, given my admittedly basic understanding of physics and crash dynamics.
I personally do state that there is no EVIDENCE that a harness is safer for a child over 40 lbs and over 4-5 years old, IF they can sit maturly. it *MIGHT* be. It might be equal..it might actually be less safe....we don'tknow,we do NOT have evidence either way.
Again,my understanding, and perhaps someone can chime in here, is that in Sweden, they actually have EVIDENCE (as the result of crash testing) that boostering is safer, which is why they choose it....but again, any testing done would have the dummy in correct position...so I think that even IF boostering is safer technically, you still have to factor the maturity criteria in, and therefore, I hesitate to recommend boosters for kids under 5-6 years old.
I also believe that even IF 5 pt harnesses are safer, it is probably by a smaller margiun, as Maedze stated..I have no evidence for this, of course, but I think it probably would be "statistically insignificant" or else pretty small..
And all of this is why I don't necesarily try to browbeat parents into EH, but I do into ERF..because we know, we have PROOF, that ERF is ridiculously safer....and in the grand scheme of things, I would much rather convince a parent to erf to 18-24 months than eh a 4 year old.