Can't even imagine how powerful Gon and Killua will be in their twenties.
We already have an idea with Gon although I don't think his raw-power will be quite that extreme. Basically, the way I think the way "Gon San" worked is that he became a MUCH more powerful version of his current self - he didn't suddenly have new abilities he didn't already know. Therefore he had a stronger Ja Janken and improved physical attributes but no newer abilities. So while at twenty years old while Gon may not have as much raw destructive power, he'll make up for it in other areas.
Killua is the difficult one because we have no "Killua-san" - we don't even really know how powerful he is right now, let alone when he's in his twenties.
This is what we know. A bloodlusted Killua without his Hatsu effortlessly one shotted Rammot who had Nen. In contrast the combined forces of himself and Gon with their Hatsu merely damaged a Nenless Rammot. This was also before he developed Godspeed.
Killua's performance against Youpi isn't much of an indicator. Him not being able to damage him doesn't say much because Youpi is the most durable character in the series [besides Meruem] - this is why its difficult to place Killua. Would those same attacks of his hurt pretty much any non Royal guard/King opponent?