If someone still doubts that his power rivals Palpatine's and Vitiate's, here is what he is truly capable of:
"Slowly, C'baoth raised his eyes to her. "You will die for this, Mara Jade," he said, his quiet voice more chilling than any outburst of rage could have been. "Slowly, and in great pain." Taking a deep breath, curling his hands into fists in front of his chest, he closed his eyes.
"We'll see about that," Mara muttered. Raising her lightsaber, she started toward him.
It began as a distant rumble, more felt than really heard. Luke looked around the room, senses tingling with a premonition of danger. But he could see nothing out of place. The sound grew louder, deeper—
And with a thunderous explosion, the sections of throne room ceiling directly above him and Mara suddenly collapsed in a downpour of gravel-sized rocks.
"Look out!" Luke shouted, throwing his arms up to protect his head and trying to leap out of the way. But the center of the rockfall moved with him. He tried again, this time nearly losing his balance as his foot caught in a pile of stones already ankle deep. Too numerous and too small for him to get a grip on through the Force, they kept coming, pummeling against him with bruising impact. Through the dust swirling around him, he saw Mara floundering under a deluge of her own, trying to guard her head with one arm as she slashed vainly at the falling stones with her lightsaber. From across the throne room, Luke could hear Han shouting something, and guessed that they, too, were under the same attack.
And standing untouched by the destructive rock storms he'd unleashed, C'baoth lifted his hands high. "I am the Jedi Master C'baoth!" he shouted, his voice ringing through the throne room and the roar of the rockfalls. "The Empire—the universe—is mine."
Luke dropped his lightsaber back into defense position, senses again tingling with danger. But once again, the knowledge did him little good. C'baoth's lightning burst flashed against the lightsaber blade, the impact knocking Luke off balance and dropping him painfully onto his knees in the pile of stones around him. Even as he struggled to get up, one of the falling rocks slammed hard into the side of his head. He staggered, toppling sideways onto one hand. Again the lightning flashed, throwing coronal fire all through the stone pile and sending wave after wave of agony through him. The lightsaber was plucked from his fingers; dimly he saw it fly over the railing toward the far end of the throne room.
" - Fully prepared Luke couldn't counter his TK in any way. Got completely overpowered by lightning. It is not uncommon to see a Force user utilizing TK and Lightning simultaniously but C'baot took it to extreme levels.
And if for some unknown reason someone still thinks that his lightning can't compare to Palpatine's or Vitiate's, here is what it can do to other objects:
"The lightning flashed again—
And with a crack of exploding metal the center of the catwalk split apart. " - His lightning can split/expload metal apart, I doubt that there is a single at least remotly comparable example by any other EU characters.
And what's really amazing is that Mara continued resisting his full power:
"Luke looked over at her. The stones were still raining down above her head; but to his astonishment, the knee-high pile of rock that had been trapping her in place was gone. And now he saw why: those lightsaber slashes she'd been making earlier hadn't been the useless sweeping motions that he'd assumed. Instead, she'd been slicing huge gashes in the floor, releasing the stones to drain through to the monitor area below.
Raising her lightsaber, she charged.
C'baoth swung around to face her, his face contorted with rage. "No!" he screamed; and again the blue-white lightning crackled from his fingertips. Mara caught the burst on her lightsaber, her mad rush faltering as coronal fire burned all around her. C'baoth fired again and again, backing toward the throne and the solid wall behind it. Doggedly, Mara kept coming.
Abruptly, the rockfall over her head ceased. From the edge of the pile that had half buried Luke, stones began flying toward C'baoth. Curving around behind him, they shot straight into Mara's face. She staggered backward, squeezing her eyes shut against the hailstorm and throwing up her right elbow to try to block them away.
" She kept resisting both his TK attacks and lightning right until Luke managed to distract him for her
to strike him down.
On death C'baot exploaded like Palpatine.
"And as it had with the Emperor aboard the Death Star, the dark side energy within him burst out in a violent explosion of blue fire"
While Luke shielded Mara from exploasion, its impact on nearest wall is tremendous, creating a hole in it:
"Luke turned and looked. The massive detonation of dark side energy had made a shambles of that end of the throne room. The walls and ceiling were blackened and cratered; the metal of the floor where C'baoth had stood was buckled and half melted; the throne itself had been ripped away and was lying smoldering a meter from its base.
And behind it, through a jagged crack in the rear wall, he could see the bright twinkle of a single star."
As conclusion I gonna point out at several facts that make these feats look much more impressive.
This fight happened just two years before DE and Luke already had 9 years of Jedi experience and fought many other more powerful users than him. Yet, he was still not even remotely as powerful as C'baot. C'baot could kill either of them at any point but instead he was toying with them until the final moment.
But the most impressive is that Mara unlike Luke in the first book of the trilogy was having hard time lifting even lightsaber sized objects. She was nowhere near to her prime. After that she became Kyle Katarn's apprentice. Even after Katarn she came to Luke's Academy for farther training. And even after that she kept improving and displayed even greater feats. Maybe she did not compare to Jacen but was immensely powerful nevertheless. She blocked extremely potent lightning. She is capable to put up mental barrier with technique she learned from Palpatine himself. She has got incredibly strong will and is one of the most staborn characters in the mythos. It makes sense now why Jacen never tried lightning againt either Luke or Mara because they could defend against immensely strong lightning long before they reached their prime.
I think she's got the right set of skills to take on Vitiate.