Extra jobs kind of misses the point... we're talking about working harder on your job in a given company, not working harder as a person in general on whatever life throws at you.
To be honest I'm not even sure how you'd measure how hard someone is working when you're comparing across different jobs, but I would find it interesting if there was any kind of data on this. I'm not sure if I'd find it more surprising if the correlation was positive or negative, but what I do know is that in an ideal world the correlation would be positive...
The better paid your job, the more unique it is, almost by definition: limited supply means the price of labour is higher. Well paid jobs aren't very fungible.
Even within a single company, people with the same title don't do the same work. And working smarter - aligned with the company goals - still pays much more than longer hours.
To be honest I'm not even sure how you'd measure how hard someone is working when you're comparing across different jobs, but I would find it interesting if there was any kind of data on this. I'm not sure if I'd find it more surprising if the correlation was positive or negative, but what I do know is that in an ideal world the correlation would be positive...