Shifters have a greater influence on the feel and control of the shift than ever the derails do. A nice RD will give a sense of lightness/quickness to the shift, but the shifter will give you the smoothness and precision of the action.
Put another way all derails pretty much go where the cable tells them to go. Shifters control the cable.
Nice derail on crappy shifters will result in crappy shift feel and movement. You can not overcome the sins of a bad shifter with a nice derail.