Mode S does show the actual IAS not the selected speed, so we can see what the a/c is actually flying although it's not mandatory to check, I believe that if I tell an a/c to fly 160 to 4 DME, and they read it back, it's not unreasonable to expect they should do it! Or if they can't to tell me. 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.
Regarding jets that don't slow down, personally I think this is less of a problem because, as long as separation with the preceeding isn't lost, it'll be the one who doesn't comply with the speed restriction that goes around, so they'll learn the hard way. For me the greater injustice is when an a/c slows down earlier than 4 DME and consequently the following a/c, who does comply with the speed restriction, goes around. Sometimes I feel as though the best thing to do would be to file on a/c that cause go-arounds in such scenarios although I think I'd probably spend most of my day on the computer filling out forms rather than actually controlling!