If you want best performance out of a 327 magnum, you have to load for it. In terms of self defense effectiveness, you need to have good penetration with adequate energy. Given the same bullet construction, the projectile with the greatest sectional density will penetrate the best.
The most common comparisons for the 327 are the 38 special and 357 mag. Since it was mentioned in this thread, a 38 special 158 @ 850 generates 254 ft/lbs of energy with a sectional density number of .177. Because sectional density rewards heavy bullets relative to bore size, one of the largest common 357 magnum is a 180 grain. Federal lists two 357 magnum 180s one at 1080 fps and another at 1130. These two will have 466 and 510 ft/lbs of energy respectively and both with a sectional density of about .202. I load a 327 magnum 153 cast bullet that now makes 1190 fps which generates 481 ft/lbs of energy and with a sectional density number of .225. This puts the energy right in with the 357 magnum plus a bullet that penetrates better. The 38 is not in the same picture. Just for comparison, a 45 Auto 230 grain (Federal Numbers) would be in the 900 fps range for 414 ft/lbs of energy with a sectional density number of .162. Another 327 magnum benefit is significantly less recoil.
The "problem" with 327 magnums is lack of factory ammo options. Most all it is in the 85 to 100 grain range. This creates three shortcomings; low sectional density numbers, thinner jackets on the bullets that expand quick and maybe too quick for 327 velocities, and too much powder to burn in very short barrels. YMMV