"In 1988 DoD changed the payload requirement from 172,200 pounds to 167,000 in order to accommodate the addition of a 4-pallet ramp and OBIGGS that added 5,000 pounds additional weight to the [C-17]"
http://www.dau.mil/pubs/arq/94arq/batte.pdf
Not clear how much was the ramp and how much was OBIGGS (sure and you'll find the man down the pub, with his mate O'Boggs) but OBIGGS is not a trivial piece of equipment on a transport-class aircraft. And by the way if it's onboard it needs to be maintained, and it does have a safety aspect because along with the I it generates O, and you don't want extra O leaking all over the place.
So adding the plumbing is probably smart. Then you can figure out whether none, some or all of your fleet need it.