The SI is probably the weakest in the force. To make up for this he cheats. He goes around absorbing other sith's energy and claiming it as his own.
1. Nice necro.
2. The SI defeats Darth Skotia and Darth Zash before even binding any ghosts. The SI's Force potential was noted to be immense a couple of times, and the SI, as noted in one of the posts above, didn't receive proper combat training as a slave, yet was still strong enough to pass the Trials easily.
3. The SI has the choice to release the ghosts and keep every ounce of power. So this basically implies that the ghosts only served the SI as a shortcut to releasing their full potential. It's like using the Hyperbolic Time Chamber in Dragon Ball (
). Well, and as a matter of protection from the arcane Rituals Thanaton had knowledge of.