well if you see, most of the trains which agot converted to Superfast ae already running with 52/53kmph ..with minimal speed..they are able to achieve 55kmph which is of superfast category..
for SWR and SCR, not many express trains enjoys this kind of speed...generally it will be below 50 and the long distance trains which are above 50 needs approval from all zones to make it a superfast ..so that's why you wont see much in these zones