I started to track something like that because I asked the same exact question. I questioned it a while back when there were (I cannot recall correctly) something like5 LEO's who were shooting at armed suspect who had taken a bus hostage. I cannot remember what the number of rounds fired by the LEO's was, but it was rather high. The actual hit on target number was low -- there would be plenty of factors for that though.
I actually took that line of thinking over to officer.com at one point and tried to get some feed back but was met with some pretty stiff resistance. I will say that having shot with a particular LEO at a range and seeing some fire marshalls shoot they aren't all bad shots. My original intent was to find out how often the LEOs were funded by the department to shoot, compared to what they had to do on their own.
The other thing I wanted to learn, was how often they had to qual and what the qual test was. Given that data one could surmise they could practice and perform the same tests to determine what their own skill level was. That's alot of digging though.
It also lead me to track more closely the events in my community. I read the policepress releasesdaily. I then began tracking the violent crime types and compared it to other similar types of locations - city, population size etc to see if a place with more stringent gun laws were higher in violent crime. Not surprised by the result.
However after formatting my hard drive a few times I no longer have any of my data and I do not have any time to dedicate to recreating all that. It's out there if you want to, but it's a heck of a job. I only did one years worth, and that took some doing.