That's one big problem I see with performance based pay, even if an accurate personal metric were available. Time spent helping/mentoring others would have a negative impact on a senior programmers bottom line, even though a company generally wants to encourage this.
You can't be accurate when you need to hit multiple different targets with one shot. It's exceptionally difficult to get people to agree on what's valuable, and how valuable it is.. and the problem gets worse with scale.
