Hi
I don't have the stats, but I would guess that if they were separate lists, less people would be on the announce list than on the test list, and there's already a deficit of people using updates-testing. To increase that, announcements should hit as many people as possible. Perhaps posting them to redhat-list, and fedora-list too. ;) Ok, maybe that's too much.. :)
Well, the reason I'm not subscribed to fedora-test-list is that there is way too much going on there leading up to test1 and going through until release for each release. If fedora-updates-testing-list & fedora-test-list (for testX releases) were separate, that might help get more people on boeard for update testing.
It's pretty easy to set up filters to check for announcements based on the subject or the sender.