Approx. how much more H.P. do you ge with an aftermarket exhaust?

It really depends on the exhaust, the size of the engine, and how efficient the stock exhaust was. The exhaust doesn't "make" more power it just releases the power in the engine. It does this be freeing up the back pressure in the exhaust. Think of it like trying to drink a cup of soda through a coffee straw, and then a regular straw. You have the same sucking power, but the bigger straw allows you to get more soda. In the same way the engine is trying to push the exhaust out a small hole. So when you give the engine a bigger hole to push the exhaust out, it has an easier time doing it, and as a result can make more power.

As for numbers anywhere from 5 - 25/hp. Again depending on the size of the engine,and how efficient the stock exhaust was.

 

Do a search for dyno #'s. I remember a couple of years back, Rob had posted some of his 328 with various bolt on changes including the muffler