The Northern California Fly Fishing Board

I can't put my fingers on the studies but as I recall, mortality was in the lower end of the range you mentioned, like 5 or 10%. Based on what I've seen in smaller, heavily fished streams like Silver Creek and the Little Truckee, it isn't much. In fact, the agencies' biologists believe it has so little effect on the populations that they recommended additional winter season on parts of both streams. Such regs have been in effect on Silver Creek for some years with no apparent deleterious effects on population.
However, I suspect the mortality numbers are probably higher where bait and/or treble hooks are involved, as has been true in some locales. That might be part of why entities like "Peaceful Parks" made the recommendations they did.
My general "best practices" is single barbless hook, land'em quick, handle 'em as little as possible (don't even take 'em out of the water), and most of them seem to do fine, even in heavily fished waters.