Perhaps the issue is exempt employees? I don't see why I shouldn't be paid hourly.
I'm tempted to say I'd rather be paid per bug/feature, with the pay rate increasing based on the complexity of the bug/feature, so that when I think I have enough money I can say no to additional work without repercussions -- but workers in other industries organized against [piece work](https://en.wikipedia.org/wiki/Piece_work) for a reason.
http://blog.klocwork.com/wp-content/uploads/2009/10/dilbert-...
So as an employee you want to ensure you have a hard limit on how many hours you're expected to work (40 for regular jobs, or as in this case, less than that).
It's probably misguided to aim for highly quantitative measurements. If productivity is measured as output per input, you have a lot of flexibility in how to define the relevant output - e.g. hitting well-defined product goals or milestones (versus e.g. lines of code, which seems wrong).
Basketball has had this problem for a long time. The plus/minus metric was invented as an attempt to address all the things players do while on the floor, but do not directly relate to an easy to capture metric. It also tries to address players who put their own personal productivity over that of the team.