he started dramatically below luffy. koby in the first few chapters was below luffy as a child in the jungle. his growth rate is literally faster
It really isn't.
Yes, he started dramatically below Luffy, but right now he's arguably even more below current Luffy than he was back then.
Remember that Koby is nowhere near Kaido level and even a Luffy that clapped Katakuri and Doflamingo (both characters who would toy with current Koby despite his growth) was fodder against Kaido.
G5 literally boosted Luffy's power x10 at least and that's a conservative estimate.
All Koby has shown so far is solid haki mastery (no CoC btw) and one ability that resembles the King Punch from Elizabello.
That's it. Right now that's not even VA material.
Luffy will be God tier at eos, so Koby reaching admiral level at final war isn't a huge problem as long as he gets CoC & a df
Sure, let's just grant Koby fan fiction powerups that clearly don't fit his character at all.
He is literally the opposite of an emperor and was a massive coward to begin with. CoC is a power that only a very small fraction of the bravest and most daring people by nature (!) have from birth on. It makes zero sense from a writing perspective that Koby has it.
A DF would also devalue his whole character arc so far (achieving this much without any shortcuts, but simply due to super hard work).
This way I could argue that Nami will be God tier by the end of the series if Oda just gave her CoC, her own DF ability and a solid training arc. Makes just as much sense.