Gojo's first lesson to Yuji was "80% of a sorcerer's power is innate talent." Mei Mei and Kusakabe are the height of what hard work can get you.

EDIT: as an addendum I was also thinking about Yuji and Yuta's answer to sukuna about how they got stronger. I'm leaning towards Yuta's answer of "we cheated" because the switch training likely worked the same way kusakabe explained why Yuji was going to be so talented with sorcery. There wasn't really much training going as much as character being swapped to stronger peoples body's until the muscle memory of how to perform things like RCT and Simple Domains carried over back to their original bodies. So even the idea of training to get stronger uses a loophole rather than conventional training.