I've read somewhere
that the waist gunners took the biggest hammering, I'll see if I've got it here rather than something I read off a web page
I believe in this book there is some numbers, "Gunner" ISBN 1-55046-332-2.
From memory, it is as you say the waist gunners, with the ball turret gunner being one of the safest crew positions. Again from memory, iirc, waist gunners suffered about twice the causalities of the next crew position but then there was usually 2 waist gunners.
The book had other interesting info like:
Testing done by the USAAF found that the bullet pattern from a B-17 during ground testing had the following results for 12 rounds to 600yds:
ball turret > dia. 15' - 8.3mils
upper turret > dia. 21' - 11.7mils
chin turret > dia. 23' - 12.6 mils
waist(closed) dia. 26' - 14.3mils
side nose > dia. 34' - 18.7mils
tail turret > dia 45' - 25mils
For the B-24 it was:
ball turret > dia. 15' - 8.3mils
upper turret > dia. 20' - 11.2mils
nose turret > dia. 23' - 12.9mils (Emerson)
nose turret > dia. 35' - 19.3mils (Motor Prod.)
waist(closed) dia. 23' - 12.9mils
waist(open) dia. 63' - 35.6mils
tail turret > dia 35' - 19.3mils
There was also data on the position attacks on the bomber came from.