Speed has a very big effect on wounding, moreso than sectional density. That's why rounds like .204 Ruger, .223 Remington, and others are effective.
Sectional density just translates into inertia. Speed equates quickly into energy, which is delivered via hydraulic shock on the body. That's why when you hit a jack rabbit with a .204 Ruger in the chest, it's head shoots about 10' in the air and it's chest cavity turns into pink mist and meaty bit shrapnel. Shoot the same rabbit with a .45 or 9mm and all you get is a nice 1" hole. Both are effective in their own way.
Birdshot is just not effective because it lacks inertia (low mass which means poor sectional density), and sufficient velocity and geometry to create hydraulic shock. It's like shooting someone with a pellet gun 300 times, LOL.
Hit center mass, odds are the person would die from all the holes and overall trauma, but it wouldn't be immediate.
I would not recommend bird shot beyond shooting someone in the face. A face shot would blind someone, if not kill them. This is why some guides will tell you to load birdshot as the first round against dangerous animals like bears; for the same purpose, blinding it, while you unload slugs into it.
My typing kind of came out wrong. I know speed is important. I was trying to say that because of the mass of bird shot, the relative speed of hot load and standard loads is rather inconsequential.