Indeed every CPU is different, for example I have a stock Yikes! 350 MHz G4 CPU running flawlessly at 400 MHz in a B&W G3 (I think it might even have originally pulled from the Yikes! running stock at 400 MHz, at least it's how it was sold to me but who knows, might be that the seller got confused by Apple's "strategy" to address the lack of 400 parts back then was to slap in 350 MHz parts for the same price)
However, on the other hand I have a 550 MHz OWC Mercury G4, part-rated at 550MHz that is, refusing to go to 600 MHz and even buggy at 550 MHz (but okay at 533 on a Beige G3 or at 500 MHz on the B&W). I have however read somewhere that this is a common bug on the B&W (it even has a name which I don't remember) and could be corrected by adjusting the voltage directly by changing jumpers on the ZIF CPU module (via soldering).
I thus run it at 500 MHz with Leopard (thanks to the Complete Leopard on Unsupported Macs package and https://web.archive.org/web/20130924163750/http://www.mactech.com/2008/09/23/leopard-pre-agp), the other option is to remove R811 at the back of the mobo (https://web.archive.org/web/20071022015429/http://www.bekkoame.ne.jp/~t-imai/g3ce2.html) however I have read that this is quite unstable (https://www.applefritter.com/node/4289). Can anyone confirm?