Any speed restriction variation from the ICAO standard is in the country specific AIP. If you Google search HK AIP, you can view their AIP and the airspeed rules are in the Enroute section, EN1.5 for VHHH.
HK gives the ATC controllers the authority to cancel the 250 below 10,000 restriction. In the U.S., ATC cannot cancel the restriction, the FAR mentioned allows acceleration to clean maneuvering for performance, but that is the limit.
My understanding is if HK cancels the speed restriction of 250 below 10,000, then you can accelerate per your discretion.