The people above are correct. An o/r mid pipe would be your best and cheapest source of "power". The best "bang for the buck" and best "Seat of the Pants" increase are gears. Don't fear the gear, 3.73, 4.10s, or if an auto, even higher....like 4.30s.
For the exhaust, a cat-back exhaust really doesn't add much power. It opens it up just a little because you go from 2.25" diameter exhaust pipes to 2.5", but that doesn't help much on a stock powered car. The cat-back just give you the tone that your car will have. The restriction is not in that part of the exhaust, it is in the 4 catalytic converters on the mid-pipe. That is where the power gains will be made. There is a drawback to removing 4 cats, it gets louder. Especially with open types of mufflers (Magnapacks, SLP Loudmouths,
Borla Stingers). I am currently running an H-Pipe with the Steeda Stainless exhaust, and I think it sounds great. Not too loud when normal driving, but it can get quite loud when I get on it.
What people like for tone is as different as what people like to eat. I like an old-school muscle car tone, but not too loud when just driving normal. I had SLP LMs, and the drone at 2500 rpms (my highway coasting speed) was retarded. It gave me a headache just driving 30 miles to work. Now, with a chambered type muffler (Flows, stock type) you can keep tone, but lose some of the raspiness and all out volume of the "glass pack" types. My Steeda's are a combo of both, and overlooked by most people. The chamber shape of the muffler still has a 45 degree straight pipe through it. It just kills a lot of the raspiness and volume at lower speeds. When I get on it, it cuts through the straight pipe, and almost as loud as others on the market.