In fact, storm bolter might be _weaker_ on combat pistol, due to knockback coming into effect at 12 damage. If the first bullet causes knockback, the second bullet might simply fail to target the knocked-back creature and automatically miss.
With SoG3 (a standard for pistol builds) this is not an issue for standard pistols, causing 11 damage per bullet at most. With a combat pistol, the maximum damage is 12. And that's not counting ranks in SoB, which stress the problem even further.
To say it in a diplomatic matter - in my humble opinion, storm bolter is one of the less useful assemblies.