The slides are lined with a material that brakes the slide, so once you reach terminal velocity you won't be accelerating anymore. You'll just have a burned tush. Better than a broiled body anyway.
As has been stated, the 744 has upper deck slides too. Also, the number of pax per door is not markedly different from the 744. I did the math once in tech ops, but can't be bothered to look it up.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo