Conventional wisdom is that the survival rate is better leaving the hook in, and that it rusts out. There may be studies to back this up but I have never seen them quoted- perhaps some of the pros on this site know what they are. My thoughts are that if you are already fishing catch and release you should be crushing the barbs down to make it easier to get the hooks out anyway, since you are worried about fish getting away. I am a surgeon and I have found that I can remove barbless hooks from the esophageal opening by sliding a hemostat through the gills and releasing the hook. This approach gives you an angle to turn the hook and release it that you cannot get through the mouth. Smooth cold steel seems pretty atraumatic to the gills.
I do not have an answer for hooks that penetrate the gills and produce bleeding. I have been told that these fish generally die. Again, this is all anecdotal. I have not seen studies documenting a high mortality with gill hooked fish.

Layton Runkle