There still seems to be a lot of grey around the area of STM limits on the new service. I've seen some people who claim they were still limited after 800MB of downloads, even after receiving the 10Mb upgrade!
I've been on 20Mb since it came out and do get 17Mb out of it during most off-peak times so I can't complain but, if I'm honest, 10Mb is enough for me whereas 4Mb wasn't and so I might consider downgrading.