FWAAA,
I don't know the numbers off hand. there are domestic and international differences as well as old contract/new contract ones plus FAA regs. It may have dropped to 10 hours of rest, but I'm not positive on that.
You did touch part of the issue. It is of course variable how bad one's experience may be and affect the ability to get rest before duty while deadheading. One person might be 5'5" and 125 lbs, get an aisle seat near a window, have the seat next to them empty or one of the bulkhead seats with adequate legroom. Not a bad experience. Another person might be 6'5" and 250 lbs, get the center seat with two beefy 250 lb pax sitting on each side on a full flight that had a delayed takeoff, late or major headwinds along with significant turbulence preventing some inflight leg stretching. Toss in a bad back and a noisy hotel for the 10 hours of "rest", and the person is DOA for the next leg.
More than a few "interpersonal" problems occurred when crews were deadheading. I for one and greatly relieved that we will be out of that UFC cage match when it comes to deadheading.