When GM designed the exhaust system, they did a pretty good job creating a stainless steel system that worked and produced nice tones. Since these systems are now 16-20 years old, they are in need of replacement. You can do anything you want because it is your car. The cat is for emission control to burn up unburned hydrocarbon emission. Do you really need it? Probably not if your car is tuned properly, but if you care about the emissions it would be a good thing.
You can go to Midas and get a cheap muffler and pipes that will last a year for $160. Then next year you can get another, then another. Each costing $160 for the pipes afterall the mufflers are crap and are free.
You can get another stainless steel system exhaust for your $500 that will last for 10 years. These systems are designed for the Fiero and are not pieced together parts so that the exhaust just exits the back of the car.
The amount of HP gain or loss from different exhaust systems don't have a lot of real dyno data to support what is the "best". Most of the proponents of any specific set up tell us the "seat of the pants test" says which is one is better than another. That is just BS. You wont feel 5 HP difference one way or another.
Again, it is your car and you can experiment all you want for your "best" exhaust system. It all takes $$.