I'll leave your questions for Murasame to answer since Murasame is the specialist in this area. But I think you're getting confused with the definitions.

8 kbps = 1 KB/s

If you were getting 30 KB/s then that's equivalent to 240 kbps. Which is not unusual b/c there's always overhead loss. Yeah about 50 KB/s would be about your maximum possible I'd suggest - perhaps just over.