It won't really add any HP, it's ment purely for asthetics. IMO it's not worth it unless you're absulutely in love with the way it looks. I was considering it for a while (long ago when I knew little...) until I was elightened by Modded's finest:hug
You can get gears and a tuner for a little more then half of what the shaker would cost. 4.10 gears would be the best bang-for-buck mod you can do at this point. Install will probably cost you about $200 or so, but that will still be less then the shaker and MUCH better on a performance asspect.