Never used a 50 foot chiller. From what I have heard and understand about science, by the time the cold water gets 25 or so feet through the chiller it has already exchanged as much heat as it can. 20 minutes doesn't seem bad compared to my chill times. I get from boil to 75-80 in about the same time, maybe a little more. I bring it the rest of the way in my ferm chamber.
For what its worth my plan, when I upgrade to a keggle for my BK, Im turning my current 25 foot into a prechiller and buying another for the kettle. I personally would rather go that route than an expensive plate chiller and dealing with cleaning and sanitizing of it.