To the best of my knowledge, we don't provide ATC with a TAS, we provide them a FL and a Mach Number. Maybe your system converts that to a TAS by the time you see it, and if it does there's the start of the problem.
My airline (QF) has standard descent speeds - for most fleets it is 280kt. We are told that ATC knows that. I have never witnessed a fellow crew member intentionally descending at a speed other than 280kt unless requested or authorised by ATC, so I don't understand the comment that you never know what speed we are doing.
And why is it so difficult to just request a certain speed from an aircraft if that will make your job easier? They do it in many busy airports elsewhere in the world. Sydney does it sometimes, I've been asked "make your descent speed 280kt" several times... I don't understand why, according to Qantas you have been advised that 280kt IS our descent speed unless authorised otherwise.