First, we must know What power Jerec was searching in the valey. It's about 15 years since I played this game, so I will rely on starwars.wikia article on Jerec, Apotheosis section for this.

Using the Force, he opened the dome and leaped inside to the absolute heart of the Valley's power. There, he engaged in meditation, hovering cross-legged among the captive spirits of the Jedi and Sith of ages past as he bent them to his will and leeched their strength. Reveling in the Force energies of the Valley, he reached a state of near-apotheosis in which he gained total omniscience. At long last, total knowledge—something which he had coveted for countless years—was completely, fully in his grasp, and untold power came with it.

I don't remember the details, but he somewhat accessed power of the Valley. Omniscience is a great power, but this had not saved him from Katarn.

Would he have he become more powerful than Palpatine or Abeloth? It's had to tell, has not much is know about the power the valley grant. We cannot put them in a cage and watch to see who end victorious. It's pure speculation, but my feeling is he would have hold it's end against Palpatine, but he would have not stand a chance against Abeloth.

Would he shall become more powerful than you can possibly imagine? Probably, from a certain point of view, but I have a bad feeling about this....

First, we must know what power Jerec was searching in the valley. It's been about 15 years since I played this game, so I will rely on starwars.wikia article on Jerec, Apotheosis section for this.

Using the Force, he opened the dome and leaped inside to the absolute heart of the Valley's power. There, he engaged in meditation, hovering cross-legged among the captive spirits of the Jedi and Sith of ages past as he bent them to his will and leeched their strength. Reveling in the Force energies of the Valley, he reached a state of near-apotheosis in which he gained total omniscience. At long last, total knowledge—something which he had coveted for countless years—was completely, fully in his grasp, and untold power came with it.

I don't remember the details, but he somehow accessed the power of the Valley. Omniscience is a great power, but it could not saved him from Katarn.

Would he have he become more powerful than Palpatine or Abeloth? It's hard to tell, as not much is know about the power the valley grants. We can't put them in a cage and watch to see who is victorious. It's pure speculation, but my feeling is he would have held his own against Palpatine, but he would not have stood a chance against Abeloth.

Would he have become more powerful than you can possibly imagine? Probably, from a certain point of view, but I have a bad feeling about this...