This is a very good question. There was a fairly scary piece on this a UK TV show Fifth Gear...the piece on third row seats is about 39 minutes in.

Fifth Gear Season 15 Episode 1 airdate 01/05/2009- Video

They show a 7.5 ton lorry hitting a stationary car at 40 mph, it doesn't make pleasant viewing. Admittedly the car is smaller than an X5 and the seats are nearer the rear window.

Toyota has developed an air bag to help with rear impacts

Toyota develops new rear window airbag for small cars