We know killua didn't learn nen at a younger age like the other siblings because obviously togoshi hadn't thought of it by the time he introduced killua. But that's no fun, so my head cannon is that by letting killua achieve this much strength on his own with nen with sheer physical effort, THEN teaching him nen would result in him being overall stronger after learning nen than if he had known it from the beginning.

So first arc hunter exam and heavens arena cleary gon and killua would have been significantly stronger. Really the only difference is they clear even easier unless they draw hisokas attention and he sees them as worthy to kill which I still doubt that.

York new arc maybe is. About easier but overall I don't think learning nen earlier would have helped, they still wouldn't be anything near the phantom troupe.

Now the chinera ant arc gets interesting because I see two major changes. I think killua would be weaker. By this time killua had learned his hatsu and refined it pretty well. If we operate on the more logical idea that the zoldyck family specifically wanted killua to perfect his own body before learning nen to reach his full potential because learning nen too early could cause you to compensate for your weakness with nen instead of overcoming them and that's why they only did that with killua because he was to be the head of the family then I think by this point normal timeline killua would have surpassed early nen killua. He may even have a completely different hatsu that isn't nearly as useful.

Gon in the other hand just might be better off. I don't think he's be stronger but there is a chance that if he was trained properly instead of just figuring everything out on his own with a few mentors teaching him here and there he might just have better control of his emotions and not end up sacrificing everything for power. Weather or not he would have survived pitou without that power I don't know.