Im pretty happy with my HKS Super Dragger. I have a Hi-Power on my Nissan and its pretty loud.
The Super Dragger has a nice tone, but isnt droning on the highway. Its loud when the car first starts up, but once it is warm you dont really notice much it at idle (which is good for not drawing attention or driving through the neighborhood late at night).
I dont have first hand experience with the Hi-Power on a supra. I have seen a number of other models/brands though that were quite loud.
I think the super dragger is quieter because of the large diameter of the muffler case. It allows for more sound dampening. A muffler that doesnt have much material wrapped around it is going to be louder.