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So if I wanna gain some top end speed or horses, I must put on the dual exhaust that comes stock on the Grand Am GT or the Grand Prix?  I wanted to get dual, but I had a couple of questions about it, like would it lower my compression ratio?  I'm just curious b/c I put a 2.25in piping on my exhaust from the cat back to the muffler, but I wuz wondering if it would be beneficial to put on dual exhaust.

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So if I wanna gain some top end speed or horses, I must put on the dual exhaust that comes stock on the Grand Am GT or the Grand Prix?  I wanted to get dual, but I had a couple of questions about it, like would it lower my compression ratio?  I'm just curious b/c I put a 2.25in piping on my exhaust from the cat back to the

NO, the dual outlet exhaust on the grandam/pri are mainly for looks. They just have dual after the rear axle. You wont see much difference with that setup. A TRUE dual might gain you some top end HP but would be very hard to do on a Beretta.
If you already have 2.25 from the cat-> muffler, I would say thats good enough if you have a highflow muffler. A highflow catalytic converter helps also.

What about the possibility of getting 2' piping for the y pipe? How about running dual with crush bent pipes? Wouldn't that have the same benefit as having one free flow muff with 2.25 piping?  

most likely it woulod be close to that or maybe 2.5" piping....and in reference to wondering about puttin a 2.25" catback system on....you will see some gains, but not much....it will however help when and if you do decide to do some engine mods.....

If I wanted to put on dual exhaust, it would only be for the looks & not performance, like on the Grand Prix?

It's my obsevation that for a street driven car with 2 valves per cylinder, 6k redline, and less than 200 cubes, a 2.25 in. single exhaust with good muffler and cat should be MORE than adequate. Torque is what is most effective in a street motor. By opening up the exhaust, you will often change the place where peak tq and hp are made. (Most often they will be higher up in rpm) This change can be slight or dramatic depending on several factors. If you are going for "looks" the y pipe and duals will be fine. If you are going for results, be realistic about your mods, both now and in the future. On a street driven engine, it's often true that "Less is More". I hope this helps.