You can try a server that you know is consistent such as a webserver to test on. Since Youtube goes through Akamai and Limelight CDNs, it’s be hard to match all subnets that they would be on, since they could have more than your stated five.
Or, if you can, get a ip of a machine that is on your lan side, make sure it is not doing anything, check the firewall section, then connection Tracking section, put in their ip. Make sure it’s not doing much. Then load a youtube video, and see what connections are going through, get the ips of the source on the left, and go to http://www.arin.net, and put in the ip address, see if it goes to Akamai/Limelight, get the subnet, and make a classifier for that subnet.
Also, restricting speed to such a CDN subnet could result in slow downloads/uploads for other things as well, windows updates for example, or anything else that would reside on those subnets, anything legitimate to use. Limiting youtube, vimeo, google-video, etc., is hard to do by ip, it’s a pain. Try a known static webserver first to download from, to make sure the classes/classifiers are working correctly. And, if so, then more than likely the youtube stream is being fed by a different CDn that is not on your subnet list.