Jacen was able to do battle with Lumiya's Force phantoms - which Jacen proclaimed "would be a fair match for Luke". Apparently, since New Jedi Order, Jacen has progressed in leaps and bounds in terms of abilities.
That, or it's another attempt to try to make Luke appear more realistic in that he is not the Star Wars version of Superman anymore. Zahn tried that, once.
Anyways, if Jacen can engage in a lightsaber battle against that - he'd likely defeat RotS Sidious in a sheer lightsaber confrontation. But I don't know about his Force powers.
If those phantoms would be a fair match for Luke then I am done reading new SW books. Jacen being that much stronger then Luke would be horrid. Jacen even coming close to Luke would be horrid. From how powerful Jacen was throughout the first LotF book I do not consider him stronger the Sidious in the force for sure. That guy with the bomb on his back, Sidious would have just mind controlled him. The Y-wing, Sidious would have just knocked the guy unconscious with the force. What was the other things Jacen did again? Whatever they were, I can pretty much guarantee Sidious would have done a much better job. And I wouldn't put it past Sidious to be able to take Jacen in a lightsaber fight after Jacen'd fought Obi-wan and the others before him.
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Lumiya confesses to not even be on par with Vader (which puts her quite a bit below Palpatine) in Betrayal. And yet, she is able to generate Force phantoms on that powerful degree. Inconsistencies, much?
Oh, yes, Glentract, Jacen does indeed proclaim that the Force phantom that he dueled with would be capable of being "a fair match for Luke".
But, they are simply generating inconsistencies. As I said, there is a group of Star Wars authors who try to tone down Luke's "Superman-syndrome". Timothy Zahn confessed to attempting to do that on multiple occasions.