I am assuming that the salesperson wasn't talking sh!t and that the braking system does work. I am pretty sure that the mag brakes (and centrifugal + electronic) all do work to different degrees of success, however they are made to keep the spool spinning speed in line with the speed of the sinker (+bait) as its travelling towards its destination. I think I can assume that the problem might be with your expectations of the reels ability. Let me explain - all the braking systems can slow down the spool speed - the idea being that the spool doesn't "push" out more line than what the sinker needs to keep going forward. All the reels though (in my experience) are unable to know when the sinker (+bait) actually hits the water, and thats where your problem might be originating. My Shimano Torium 14 has a great little centrifugal braking system that I have set in to my usual casting weight. I can take that reel and throw as hard as I like and it will not overwind while the sinker is travelling. What does need to be done though is - as the sinker hits the water press your thumb onto the spool to stop it. Don't dab, press firmly and that should stop any overwind occurring.
If the overwind is occurring before the sinker hits the water, then you might need to play with the braking systems settings. I'm not familiar with the reel but, a web page on it I found states that, it is adjustable. Perhaps set it to the max (most braking) setting and start working your way down from there?
Maybe someone else can discuss further?