I recently upgraded my broadband in China to 60Mb/20Mb (download/upload). That is where my BitTorrent Sync server is located. I run a client in another country in Asia and have been able to get approximately 2.5-3.0 MB (equivalent to 20-24 Mb) down at the client side. BitTorrent Sync seems to be able to use my bandwidth fully.
If you are experiencing slow Sync speeds with a US peer from China, you may want to find out what throughput you can achieve outside of Sync. I ran speediest from China to multiple locations in the US and the throughput was no where close to the throughput I got running speediest to countries in Asia. Your problem may not be the Sync software.
