I had a very interesting chat with George Blankenship at the Toronto store grand opening last night, and we chatted about the seats a bit. What he says, if memory serves, is that the seat design had been (or was on its way to being) approved for the standard 30 MPH rear impact requirement, but then Elon insisted that it pass the test at 50 MPH since his kids were going to ride back there. This required a re-design of both the rear end of the car and the seats themselves. George mentioned this did add a bit of weight, and that the seat support mechanism had to be beefed up or re-designed in some way. There is a committment to deliver the re-designed seats, but I don't recall if George put a timeline on that.
Bottom line is that for those of you looking for this option, it's going to be a pretty darned safe and secure spot to stow your kids!