would a .36 caliber, 158 grain, semi-wadcutter, traveling at 830-850 FPS, hitting center-mass, be effective as a self-defense round?
And the key here is "hitting center-mass". Almost everything is effective hitting center-mass. Where HP bullets have (and show) an advantage is when shots are less than perfect.
The SWC nose profile not only punches nice clean round full caliber holes in paper, it tends to to the same in tissue. This gives it a performance edge over the same bullet with a round/pointed nose shape.
While the .38 SPL has had failures, its successes are beyond count. Bottom line, if you put the bullet where it needs to go, it will work.
One shot stop statistics are nice ways to compare relative effectiveness of rounds, but my personal belief is to shoot until the threat goes away. Only hits count, and only hits in the right place can be counted on.
I prefer large calibers, but have and carry a .38 and don't feel inadequately armed. I also sometimes carry a .32acp, and don't feel undergunned there, either.