There is no safety difference between 3 point and 5 point. (or so they say in crash testing). It's more a parental thing, I for example enjoy a 5 point harness in infant seats more then I do a 3 point, but presumably there is no difference.
As for this discussion, I would say that the way we tether and have a foot prop is very beneficial, but they are also very much required here because our crash testing is different then American, we allow far less movement of the seats then the American standard so our seats are made with that in mind.
I too am not an engineer, neither am I a car seat designer, I can only speak for what to me makes more sense, and to me Swedish tethering style is better then American, but I have never seen the two compared, so if this would be true in an actual crash I don't know. Having a foot prop is proven to be beneficial in our seats, hence why they all come with it now for rear facing, I mean the Britax TWE is in England the only rear facing car seat sold that doesn't have one but it's also not changed since our last legislation R44/03.
Most of our seats also come with a rebound bar (like the said Max Fix2 has and the BeSafe) and the izi Plus comes with tethers, footprop AND a rebound bar which in their words is "the very safest option".
I would LOVE to work with this, but sadly maths and physics was not my strongest point in school so engineering was not something I could do. hehe
All in all I think that if we took a bit from both (America and EU) we would make a heck of a good legislation and quite good car seats.