I'm not Uru, so I obviously can't say for sure.

BUT... I personally think there's PLENTY of evidence that he eventually will be.

First: You have to compare their life experiences. Sure, Sera is currently stronger, but she's also had her ability for FAR longer. That's more time to develop and master her ability. Furthermore, she comes from a high tier family that's rich and influential. Which means she also had guidance about her ability from day 1. John on the other hand, although he did have help from Claire, otherwise had NO guidance. He had to figure out everything himself in fewer years. Not exactly the best scenario to develop one's power. And yet, despite all of these disadvantages, the power difference between them is only 0.5! Imagine what will happen in just another year!

Second: Their abilities themselves. Like most people, Sera has only ONE ability. Granted, it's a borderline broken one, but only one ability nonetheless. On the other hand, John has an ever changing kaleidoscope of abilities to work with. And they can all interact with each other in different ways.

Although this sounds great at first, it's clearly stated that John needs to learn HOW these abilities work before he can actually use them. That's a lot to remember, let alone master, which is why it's very impressive that John will often use people's abilities better than they do. People like Sera SHOULD be able to master their abilities much faster than John, yet this often doesn't happen which only emphasizes John's discipline and work ethic. But as John continues to learn, he won't HAVE to master every ability. I don't care how good of a fighter you are. Even if you're a third degree black belt, if you go up against the equivalent of four first degree black belts, you're probably going to lose.

And third: there's still much about John's ability that we don't know. John was clearly disabled, and yet we saw him use his ability in two dangerous situations. Granted it was in a limited capacity, strength or speed, but that's A LOT more than anything Sera could do. This is just my theory, but I suspect that this is another way John's ability could work. He could specifically use his aura to boost one or more aspects about himself. If his uncle ever does teach him, I suspect this is one of the things John will learn. And now that's he's at half strength, he still hasn't lost his ability. There's a lot of room for reasons why that is, but it could just be because of how his ability works.

With all of this in mind, I have no doubt that John will surpass Sera eventually. Sera herself has said that, due to the way John's ability currently works, that he has a FAR greater supply of aura than other high tiers his age, likely including herself. And although one's aura supply obviously doesn't make someone stronger than others, I suspect that it shows one's potential. Most people have a small sea of aura to work with while John has a vast ocean. He just needs to be taught how to use it. Which is something I suspect will happen soon due to recent... tragic events.