With my 96' Z28 it has a flowmaster and what looks to be about 2 1/2 piping from the header back, is that stock? But my question is if I were to get dual piping from the headers to the flowmaster would that make it louder, and would it give it any more HP??

 

If you were to put two pipes into one muffler then you really won't see any performance gain, maybe a little but not much since its still bottlenecked at the single muffler. True duals--two pipes running off of both manifolds then back to mufflers would net more power. As far as sound, you can make a single pipe just as loud as true duals, the popular loudmouth is like having no muffler its so freeflowing and very very loud. My current true dual mufflers are pretty quiet, would sound like stock if it weren't for the cam, the duals give a different sound over a catback. I would look up some posts and find some soundclips to help you decide what you are looking for