I am en-route Brisbane and have quite a bit to do with this lately. The only real problems are caused by aircraft not flying at the speed the company has filed them with. I am frequently checking this now, and frequently finding enormous variation (despite being told by some crews that the company has told them to fly at the speed stipulated). I asked if it depended on whether or not the flight was running to schedule but apparently not a determining factor. One Virgin B737 plan will list a TAS of 450 and another will have 375. Even at the same level. MAESTRO allocates landing times based on the cruise TAS planned. If you go fast it will not reposition you earlier, it will just insist that you are running ahead of time and you need to be delayed to achieve your time. (It's French, after all!). ATCs need to be aware that changing the TAS in the "overall" cruise TAS field of the FDR will be overridden if route segments have any level changes and a different TAS stipulated for those route segments. (I just realised/remembered this recently!)