1. I'm pretty sure that the failures have nothing to do with the number of virus found. Finding a virus is not an operational failure but a successful execution. If you want statistics on what is being found, then your vscan server software should be able to give you those. I know the Symantec software can offer these.
2. Throttled requests refers to "requests waiting in the queue" because the vscan server is already too busy to accept more jobs.
If you are really interested in more complex vscan statistics on the filer side, the 'stats' command in diag mode allows you to view a lot of stats under the objects: "vscan_server_stat" and "vscan_stats".