keep in mind flowmaster mufflers are chambered. so they wont flow nearly as good as say a magnaflow would. so you will get more HP with a magnaflow than a flowmaster unless you put a flowmaster...
Depends, a nicer setup like GReddy or HKS, or maybe Skunk2 (my favorite) would run you in the several hundreds, but return atleast 10 hp and probably more like 20 hp, IF, your cars other parts are...