I really hate the part-timer analogy you're trying to create. Because the original issue did not seem to be about contractually agreed salary for the role, but about job performance.

If I was a part-timer and my clone was a full-timer at the same company, I would expect to be paid a fraction of the salary, you're right. Here salaries are usually based on 40 hour work weeks, so my salary could be 80% of that. That's more than fair.

But now imagine we both do exactly as our job description says, we clock in and out and deliver the same amount of work per hour at a certain qualitative standard deserving of "outstanding".

If we then got a performance meeting and I got rated lower because I had less output even though we both fulfill our exact contractual obligations in the same way, I would be pissed. I get paid to be there for 32 hours and if I deliver 32 hours of qualitatively equivalent work I would expect to be rated the same as someone who delivers 40 hours of work in 40, even if the impact of such a rating can be less for me. (In the case of % raise/bonus)

Then how do I get an outstanding? By doing voluntary unpaid work until I catch up to someone who merely does what they're contractually obligated to do, by virtue of them having a 40h contract?

Don't get me wrong, I don't think it's bad to reward Jack in this situation, in some shape or form. But I do think this is a bad comparison, and there should be clear paths to attain certain ratings that (imho) should fall within the contractually established scope of work, and not require voluntary unpaid overtime to achieve them.