This is something virtually all functional companies do when they decide raises. The fact that they don't do it perfectly isn't a huge problem, they just need to be more right than wrong.

> Do you go to customers and ask them which features provide the most value to them, and then follow the code back to the people who implemented them? Do you go to the customers who paid the most, and repeat the question to them only?

Does productivity equal sales? I don't think so. If someone does a good job implementing a feature that doesn't drive sales, that should count toward their productivity. Equally, imagine a task that could be assigned to anyone that drive sales: it doesn't make sense to reward the person who happened to be assigned this task when anyone could have done it.

You're demanding way too much here because you're unwilling to get "handwavy" and instead want some rigorous way to quantify productivity. Instead, embrace subjectivity! Imagine you're in charge and ask yourself questions like:

1. If I need to organize a meeting to address some problem that needs to be solved as soon as possible, who would I invite to the meeting?

2. If someone tells me he plans to quit, how much would I be willing to offer to convince him to stay?

3. If someone quits, how hard are they to replace? In terms of hiring a replacement and/or transferring their responsbilities to someone else.

I suppose there are some workplaces where it's genuinely hard to rank people. But my sense is that they're rare, small, and careful about hiring. Everywhere I've worked, this is not the case and I'm fairly sure this is the norm.