Well, the stock booster can only output so much pressure. Say it's X number. Once you hit X, it don't matter how hard you push, you can't get any more pressure out of the system.
With hydro boost, you can say the potential for power assist is like X times eleventybillion. Again, it's got a cap. But w/e. Anyways, it has a lot more potential for assisting you as the P/S pump puts out some stupid amount of pressure.
I've ridden in a vehicle with it. Stopped pretty good. Didn't mean it was a great mod, there's a lot more to breaking than power assist (locking up the tires isn't an effective way to stop).