I agree in general with what you say, but if you want to implement on microstrip a 200 Ohm line, this will be very thin (very small W). So thin, that firstly your response will suffer from fringing field effects (imagine a capacitor that the distance between its plates is much bigger that the area of the plates) and secondly the models that are generally used for microstrips are for negligible or zero conductor thickness (thickness<W/2 according to Gonzalez). If you design a 200 Ohm line, you will end up getting a width of some microns. Given that usually the conductor thickness is 1-2 oz, i don't know if the above assumption stands. So a correct simulation depends on the accuracy of the models that ADS uses...Generally i avoid very thin lines, to be on the safe side of the models as sometimes you cannot be sure about how they treat such extreme cases ...