Thank you to all who replied! Just to be clear, my comment about the cost was only to say that I've already priced these out, am aware of the price difference and it's not an issue, whereas safety is THE issue. The reason I asked the original question is that while it's pretty obvious that the 3-point restraint is superior to lap belts, I wonder if there is anything about the retrofitting aspect that could impact its effectiveness and/or safety. The floor of the Hawkins apparently has an extremely thick steel plate to which the seat base will be bolted, but I am still picturing the entire seat becoming a projectile. Of course in such a scenario it probably won't matter what kind of belt we're wearing...