A little nitpick; we can't extrapolate much about Gon's peak potential based on what happened in the CA arc other than him being a monster (but we knew that already).
The transformation isn't Gon at his peak, it's all the power Gon will have over the remainder of his lifespan compressed into a single moment. He wouldn't get anywhere close to that level under normal circumstances no matter how hard he trains.
I'd put Kurapika a tier below Gon and Killua simply based on how fast, strong and durable the kids are naturally. Kurapika is much more intelligent, though.
Well, I said peak/ceiling, as in his absolute best - but that doesn't necessarily he, Killua or Kurapika will be able to achieve their absolute peak via normal methods. It was just a way to show: "holy, shit Gon's a beast, and so is the other two" - because it's because of Gon's immense potential that he could make such a sacrifice.
Fair on Kurapika. I think he's roughly in the same tier, but that could be my bias speaking :P