Please ignore this message, this is a mistake from my side, here is the story:
I configured Windows 7 with a webdav over ssl mount point to my syno box (DS209+II) and while transferring a big file, I got from Windows side a 24MB/s throughput while on the DS209+II it was a few kilobytes/s, that's why I believed the monitoring was not working (but then asked myself how it could be since it uses netstat). After transfert on the Windows side was almost said to be complete (5 seconds remaining), it remained at "5 seconds remaining" for a while, since effective throughput was not 24MB/s at all!
Anyway, there are issues with webdav with my box that I'll investigate:
1. throughput for webdav/ssl is crappy
2. I don't manage to set up an uncrypted webdav mount with my windows7 box (using microsoft integrated webdav client)