The J-10A's listed max speed of Mach 2.0 could be a regulatory limitation and NOT a physical limitation. Maybe China wants to hide the top speed for strategic and tactical reasons, or maybe China's military tells its pilots to fly below Mach 2.0 under most situations to prolong the lifespan of the aircraft. The USAF and US Navy did the same with the F-15 and F-14. The F-15 possibly flew faster than the listed M 2.5 and the F-14D flew a lot faster than the listed M 1.88, but the USAF and the US Navy told its fighter pilots to limit their planes' top speeds to these listed speeds. The US military made these limitations because it wanted to stretch out their fighter planes lifespans for as long as possible. The pilots could only break these regulations under real fighting situations and during very few training purposes.
According to what I have read, the fighter pilot is playing with fire if he repeatedly hits beyond M 2.0 in aged fighter jets. Even new fighter jets need lots of maintenance if the fighter jets constantly experience speeds above M 2.0. Russian and Chinese engines are probably less durable than American and European engines.