How about measuring Forecast Calls (+ % Forecast Accuracy), Offered Calls (+ % Handled, Abandonment Rate), Service Level, Average Delay, AHT, Occupancy, Utilisation/Productivity, & Adherance. Depending on the configuration of your routing IVR Handled Calls and Repeat Call Ratio may also be applicable.
They are all generic call centre KPI's that can be applicable to any Call Centre environment. The combination of these KPI's would enable you to determine site performance, just remember to set site specific targets, as something achievable in one site may not be in another (or may not be required in another).
Maybe you could set acceptable target ranges for each measure for each site, traffic light colour code the result and then summarise the the sites and their corresponding colour for each measure, you would then instantly see how they compare.
As Joe mentioned the tough bit is setting the targets, as these are all quantative measures, maybe look at each sites history to initialise your targets and going forward adjust the targets to meet business objectives. Also remember these are only Call Centre specific measures, these measures don't look at the specialist activities of your centres.
Just a suggestion :)
Ben |