<<What would be useful is if the twr had mode S readout on their ATM, then we wouldn't have to phone them to tell them an a/c isn't flying the assigned speed and to ask it to reduce/increase accordingly>>
That's only any good if TWR knows the assigned speed! 160 to 4D may be 'standard' but how is AIR ARR to know that Number 2 hasn't 'tweaked' one...or two without telling!?
We have the tech for SEL FL, why not SEL IAS?
<<For me the greater injustice is when an a/c slows down earlier than 4 DME>>
Send the front one (the perpetrator!) around then! Their company ops will soon be on the telephone to TWR SUP to ask why, thereby providing an opportunity for them to be politely told! They will soon mend their ways if their pilots' actions have compromised their commercial operation.