After his epiphany inside the temple of Exar Kun, Kyp had emerged stronger, able to accept the challenge. If he could face this final test, Luke would know that Kyp had passed through the fire of his testing-tempered by forces as dire and powerful as those Luke himself had endured.
- Jedi Academy: Champions of the Force
About as objective as it can get.
Then there's I, Jedi:
When we had met before I had felt power in him, but now, after his experiences with the Emperor Reborn, his power had been redoubled. Physically he looked a bit haggard and worn, with the flesh around his eyes having tightened and wrinkles appearing at their corners. I knew we were the same age chronologically, but in experience he far surpassed me.
The pure arrogance and stupidity of those ideas slammed hard into me. Luke Skywalker had dealt with Darth Vader and the Emperor, even the Emperor Reborn. If they weren't monsters, monsters didn't exist. Master Skywalker was more than capable of dealing with them, which made his condition now that much more stunning and terrifying.
The despair in her voice found an ally in the fear writhing into my belly. It had never seemed odd to me that Kyp had been able to slam me into a wall because he had always been more powerful than me. Even when I felt the other presence reinforcing him and got hammered by the combination of them, I never imagined that they could be more powerful than Luke Skywalker. I had even rationalized away the dark man's ability to avoid detection as his being talented in that area, just as I was talented in the area of image projection.
Not only does Corran Horn know how powerful Luke is, in fact he makes it clearer than ever that he's more powerful than he'd ever been. (Something Jedi Academy has echoed.) Corran also confirms that he couldn't imagine anyone being more powerful than Luke. Going as far as to attempt to rationalise that Kun blocking the Force senses of every single Jedi in the academy, including Luke's, as something he just had an affinity for. As opposed to the alternative. Yet he then accepts that they were just more powerful than him. Leading him to the conclusion:
We were up against something more powerful than I'd ever cared to imagine existing.
It's all very clear, and even if you factor in later quotes, Kun's standing here is only subject to scaling adapted from later sources, not direct retcons. As per Chee, a retcon is the last resort when something cannot be resolved. This can be. Kun is in general a Sheev tier heavyweight.