Today's premium hollowpoint designs are no highly dependent on velocity to achiece reliable expansion.
The improvements in designs allow these bullets to expand at their targeted velocities. Premium defensive ammo, such as the 147 grain Winchester SXT, Golden Saber, or Hydrashok, all expand quite well at the subsonic velocities.
The 124 grain standard pressure Hydrashok also expands as well as any other defensive load out there, as do the standard pressure Gold Dots.
Yes, it was true, that a while back, the 147 grainers did have problems with expansion at the subsonic speeds. This is no longer the case, as the millions of dollars that the ammo companies spent for research and development had resulted in better designs.