I'm going to answer your questions with what I know. Since the advice is free, you are getting what you paid for...
I'm going to answer by posting your question and answering it.
"I was wondering if anyone could tell me how much performance I would get and is it true if the exhuast is too big you can loose horsepower?"
I only know the answer to the last part of the question. With a bigger exhaust you mess up the vacuum pressures and you USUALLY lose horsepower. Sometimes you don't if you know what your doing or you get lucky. But you do gain torque. Torque is the big gain for exhaust.
"Also since im going to be getting coilovers soon I was wondering if I would have to change my shocks."
No, you don't have to get upgraded shocks, but it might be a good idea. Stock shocks are fine for coilovers, but it will make them wear a lot faster than a aftermarket shock would. So what you might do is stick the coilovers (my favorite are Skunk2) on the stock struts and roll them until they crap out, then go get your aftermarket shocks.