Look into top end GPU or CPU overclocking on the desktop side of things if you want to know where RPI overclocking would end up. It's going to be the same general thing - get it as cold as possible and ram as much power & voltage as possible as cleanly as possible. It wouldn't end up looking anything like a Raspberry Pi at the end of the day, though, as the first step would be replacing the entire power delivery system (see for example products like https://www.evga.com/articles/01140/evga-epower-v/ on the extreme GPU side of things)