My feeling is that generally a group knows who sucks or is disruptive, and this is probably the most reliable data source. That's your scoring criteria. Other modifications I might make:
1. flatten the org chart for the purposes of ranking. 2. employees rank best and worst fellow employees, i.e. up to 10 best, up to 10 worst. 3. ranking is two-dimensional to distinguish between "this person is terrible and should be fired" from "this person isn't a good fit/being utilized well and should be moved". 4. Do the above and see if the results can be used to determine who should be fired. 5. If you want to repeat this process every year don't set forced quotas.