You won't hate them when you get rear-ended and they save your neck!

On the 850, the headrests have pins that need to be pulled before they will come free. Requires pulling the seatback cover up to access. Good chance that 10 years later the design has changed somewhat, but then maybe not. You should also confirm whether or not the head rest has some sort of Whiplash protection feature that "serves" the headrest forward to your head in a collision to minimize the impact accelerations. I seem to recall that it was Volvo that came out with this feature. That would further complicate the removal (not to mention add an element of danger to the procedure due to the pyrotechnic device).