I must confess to a degree of ignorance with current events in the world of teacher evaluation. I had not idea that so-called value added bonuses had gained such traction. They make fun of the mathematical equation to figure this out, but it's not that complex. They project a student's performance based on past performance and then rate deviation from that projection.
Again, I think it's good that we're rocking the boat, but this formula wouldn't work for straight-A students. Also, projecting student performance based on past performance is a dangerous endeavor. I failed almost everything in high school, but got a 1430 on the SAT's. Do we credit that to a teacher that year, or the fact that I was failing because I was a belligerent prick when I was 17?
Everyone is aware of those problems, which is the ostensible reason why the teacher's unions are against this system. While I'm sure that has a large part to play, another part that they don't want to admit is the ridiculous level of protection and nepotism that the union provides them.
I still feel that the best way to increase teacher performance is anonymous evaluations from students and other teachers, grades, standardized tests, and of course graduation rates. Computer models to track students trajectory through school and determine correlations between graduation rates, SAT scores, college acceptance, and teachers that those students have had would take awhile to program but aren't too complicated.