I teach maths, so the kids are in sets. It is really important when kids are set that there is fluid movement between sets so that kids can work at the right level, be given appropriate support and challenge.
So I have two particular kids in my class, one is brilliant and will beat his target. He should be moved up a set to access more challenging work. The other will fail to meet his target which is set too high by FFT, but is working in the correct set for his ability.
If my pay is tied to their targets, I would keep the over performing kid, even though they should move up a set, and move down underperforming but in the correct set kid, so that the next teacher down can take the hit when he misses his target. That is not what is best for the children, but my pay is on the line. No?
And in my school, the bottom set always have negative value added. This is because it's got the kids in it who bunk off school, the ones who spend half the week at college, the ones who fall asleep in lessons because their parents let them play COD all night, the ones who really don't give a shit. Who is the HOD going to allocate that set to if they know that teacher will forfeit a pay rise as a result?
And it's worth remembering that a class's results aren't just down to the work of the teacher that particular year, but a cumulation of their education thus far. A teacher who inherits a class from a great teacher is in a far better position results-wise than a teacher who inherits one from a series of poor supply teachers.
While my pay is already linked to my performance because I'm on the upper pay scale, I'm far happier when my performance is rated as how I have contributed to the department, the school, mentoring new teachers, running training sessions, and things that I can control like my planning, marking, resources. Payment by results is just a lottery depending on which classes you get, and could be used by unscrupulous management to withhold pay rises from good teachers.